Spectral and phase modulation tunable birefringence devices

ABSTRACT

The present invention describes a liquid crystal composite tuneable device for fast polarisation-independent modulation of an incident light beam comprising: (a) two supporting and functional panels, at least one of them coated with a transparent conductive electrode layer and with optionally at least one additional layer selected from an alignment layer, antireflective coating layer, thermochromic or electrochromic layer, photoconductive or photosensitive layer, and (b) a composite structure sandwiched between said two panels and made of a liquid crystal and porous microparticles infiltrated with said liquid crystal. The porous microparticles have an average refractive index approximately equals to one of the liquid crystal principal refractive indices, matching that of the liquid crystal at one orientational state (for example, parallel n ∥ ), and exhibiting large mismatch at another orientational state (for example, perpendicular n ⊥ ). This refractive index mismatch between said microparticles and said liquid crystal is tuned by applying an external electric or magnetic field, thermally or optically.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation in part of U.S. patent applicationSer. No. 17/234,341 filed Apr. 19, 2021, which is a Divisional PatentApplication of U.S. patent application Ser. No. 16/644,637 filed Mar. 5,2020, Issued as U.S. Pat. No. 11,112,671 on Sep. 7, 2021, which is aNational Phase of PCT Patent Application No. PCT/IL2018/050756 havingInternational filing date of Jul. 11, 2018, which claims the benefit ofand priority to U.S. Provisional Application No. 62/553,954 filed onSep. 4, 2017 and 62/623,566 filed Jan. 30, 2018. The contents of theabove applications are all incorporated by reference as if fully setforth herein in their entirety.

TECHNICAL FIELD

The present application relates to the field of tuneable spectrum,intensity and phase modulation devices. In particular, the presentapplication relates to a liquid crystal composite tuneable device forfast polarisation-independent light modulation.

BACKGROUND

Tuneable spectrum, intensity, and phase modulation devices, such asliquid crystals, have recently become of great interest due to theirability to provide spectral filtering, intensity modulation,polarisation control, tuneable lensing and other many applications inprivacy and smart windows, optical imaging, sensing and opticaltelecommunications. However, when it comes to practical implementationin various field systems, these devices show some serious drawbacks,such as strong incidence angle and polarisation dependence, slow speed,and narrow tuning range. These problems of tuneable and spectralmodulation devices are yet to be overcome.

There are several situations where polarisation independence of incidentlight helps increasing the light throughput, for example in spectralimaging, privacy windows operation, vision tuneable correction andwave-front sensing through the atmosphere. In privacy windows, forinstance, the light polarisation dependence is a drawback because itdeteriorates the light throughput. As a result, the need for polarisersmakes such privacy window extremely expensive and less efficient.

The present application describes new birefringence-tuneable devices,such as liquid crystals, which are built to solve the aforementionedproblems. These devices can be used, for example, in privacy windows andother photonic applications.

SUMMARY

One of the aspects of the present application is a liquid crystalcomposite tuneable device for fast polarisation-independent modulationof an incident light beam comprising:

-   -   (a) two supporting and functional panels, at least one of them        coated with a transparent conductive electrode layer and with        optionally at least one additional layer selected from an        alignment layer, antireflective coating layer, thermochromic or        electrochromic layer, photoconductive or photosensitive layer;        and    -   (b) a composite structure sandwiched between said two panels and        made of a liquid crystal and porous microparticles infiltrated        with said liquid crystal;        characterised in that:    -   (i) said porous microparticles have an average refractive index        approximately equals to one of the liquid crystal principal        refractive indices;    -   (ii) an effective refractive index of the porous microparticles        matches that of the liquid crystal at one orientational state        (for example, parallel n_(∥)) of said liquid crystal and        exhibits large mismatch at another orientational state (for        example, perpendicular n_(⊥)), of said liquid crystal;    -   (iii) a refractive index mismatch between said microparticles        and said liquid crystal is tuned by applying an external        electric or magnetic field, thermally or optically; and    -   (iv) concentration of said microparticles in said composite is        in the range of 0.1%-50% for providing optimum contrast of the        device upon switching it between different orientational states        of the liquid crystal, when said device is used as a scattering        device, and less than 0.1% for avoiding significant light        scattering, when said device is used as a spectrum, phase, or        polarisation modulator.

In a specific embodiment, said microparticles have spatial dimensions inthe range of about 0.3-30 microns. These microparticles may bedielectric, organic, ceramic, magnetic, fluorescent, semiconducting,semi-metallic, or metallic. They may have a shape of tubes, rods, hollowfibres, cones, or shells. In another embodiment, these microparticlesmay be made of photosensitive, thermochromic or electrochromicmaterials. Said photo-conductive or photosensitive materials aresensitive to one part of the solar spectrum but transparent to the otherpart. The device of the present invention can therefore be controlled atleast partially by part of the solar spectrum that generates aphotovoltage or photoresistance variation on the liquid crystal.Alternatively, it can be controlled at least partially by part of thesolar spectrum that generates a photovoltage or photoresistancevariation on the liquid crystal, and partially by temperature of theenvironment.

In a further embodiment, the melting point of the microparticles ispreferably less than that of the liquid crystal. In yet furtherembodiment, the device of the present invention is atemperature-controlled scattering device designed to scatter light abovea certain temperature upon applying the external field, wherein saidtemperature is a melting point of the microparticles.

In a further embodiment, concentration of the microparticles in thecomposite is less than 30%, preferably less than 1.0%, more preferablyless than 0.1%. In yet further embodiment, the microparticles may bemade of oxidised or nitrogenised porous silicon, they may be formed bycoagulation of nanoscale particles, and prepared lithographically, bylaser patterning, embossing or by chemical etching, and randomlydistributed on at least one of said panels. They may be furtherfunctionalised with an organic or inorganic molecular nano-layer.

In another embodiment, the liquid crystal of the present device has anegative dielectric anisotropy and aligned perpendicular to the plane ofthe panels. In still another embodiment, the liquid crystal of thepresent device has a negative dielectric anisotropy, and the compositestructure further comprises more than 0.5% of surfactant material havinga melting point preferably lower than that of the liquid crystal.

In a certain embodiment, said panels are made of transparent material,such as glass or transparent flexible material, and coated with atransparent conducting oxide to form said transparent conductiveelectrode layer. This transparent conductive electrode layer may bepatterned into pixels and said external field is applied to each pixelseparately or to a group of pixels, thereby resulting in said devicebeing pixelated. Each pixel of this pixelated device is adjusted tomodify polarisation, intensity, or phase of an optical beam, therebyturning said device into a spatial or polarisation light modulator forimproving image quality and creating polarimetric or ellipsometricimages. In this device, said panels may be further coated with a thinpolymer layer to form the alignment layer over said transparentconductive electrode layer for providing a mono-domain alignment of theliquid crystal molecules. The alignment layer of the device may be madeof random liquid crystal micro-domains by nano-patterning of saidtransparent conductive electrode layer or by photo-alignment of thephotosensitive polymer layer. The neighbouring random liquid crystalmicro-domains of the alignment layer have a large refractive indexmismatch between them at one orientational state of the liquid crystaland nearly full matching at another orientational state. Said refractiveindex mismatch is tuned by modulating the external field strength,frequency, or duty cycle.

In a particular embodiment, the microparticles are preferably porous andpreferably with porosity not exceeding about 75%. The optimum porosityof the microparticles is determined by the effective average refractiveindex of the microparticles infiltrated with the liquid crystal. Therequirements to the effective average refractive index which guaranteethat the high contrast of the device of the present invention will beachieved are stated above and can be summarised as follows:

-   1) The effective average refractive index must be close to the    refractive index of the liquid crystal molecules in the    micro-domains surrounding the microparticles at one orientational    state (so that the mismatch is minimised); and-   2) The effective average refractive index must be close to the other    refractive index of the liquid crystal molecules in the    micro-domains surrounding the microparticle at another orientational    state of the molecules, governed by the applied external field (so    that the mismatch is maximised).

Microparticles having low porosity or no-porosity at all may be alsoused in the device of the present invention, but this can affect thecontrast of the device if the refractive indices of the microparticlesare not matching one of the principal refractive indices of the liquidcrystal at one of the orientational states. Therefore, some porosity ofthe microparticles can always help in meeting the criterion mentionedabove for obtaining the high contrast scattering-based device, and italso helps in selection of the microparticles with a specificallyrequired refractive index.

In a further embodiment, the photosensitive layer is an intrinsicphoto-conducting layer or a photodiode layer, and the refractive indexmismatch between said microparticles and said liquid crystal isoptically tuneable. In yet further embodiment, the panels are partiallytransparent and said device is of a wedge type. The panels of thewedge-type device may be coated with a transparent electrode layer andwith an alignment layer, and the said device is designed to producecolour interference bands on the facets of the wedge, said colouredbands are selected serially using a slit aperture tuneable with theapplied external field and correspond to the maxima in transmission orreflection. The wedge may be spherical, and the produced colour bandsappear as the colour concentric rings on the surface and are configuredto be selected using the annular aperture. In a particular embodiment,the device of the present invention is used as a tuneable birefringentor tuneable phase plate.

Another aspect of the present application is a polarisationliquid-crystal retarder system for obtaining polarisation-independentphase or spectrum modulation, comprising at least one device of thepresent invention and at least one polarised beam splitter installed inat least one channel, said beam splitter is configured to split apolarised light beam into two orthogonally polarised and collimatedbeams, and said system has a Mach-Zehnder interferometer configuration.This system may comprise two devices of the present invention and twosaid polarised beam splitters installed in two channels for two-channeloperation of said system.

In a further embodiment, the polarisation liquid-crystal retarder systemcomprises the device of the invention, a polarised beam splitterconfigured to split a polarised light beam into two orthogonallypolarised and collimated beams directed into two arms, and twopolarisation mirrors installed in said arms, said arms are glued with arefractive-index matching glue to the sides of the polarised beamsplitter, and said system has a Michelson interferometer configuration.In yet further embodiment, the polarisation liquid-crystal retardersystem comprises the device of the invention, a polarised beam splitter,and a retroreflector or polarisation conversion mirror, said beamsplitter is configured to split a polarised light beam into twoorthogonally polarised and collimated beams, and said system has aSagnac interferometer configuration. In another embodiment, thepolarisation liquid-crystal retarder system comprises the device of theinvention and two Wollaston or Rochon polarisers, wherein said twopolarisers are configured to split a light beam into two orthogonallypolarised beams with a small angle between them. In still anotherembodiment, the polarisation liquid-crystal retarder system comprisesthe device of the invention and two dielectric mirrors or twopolarisation conversion mirrors, wherein said system has a Fabry-Perotinterferometer configuration.

A further aspect of the present invention is an imaging system forminimising an angular dependence of a phase retardation comprising thetwo devices of the invention, said two devices are anti-parallel alignedand positioned at 180 degrees to each other.

Yet further aspect of the present invention is a tuneable filteringsystem for filtering an incident light beam comprising at least onepixelated device of the present embodiments, wherein said device isintegrated within a polarisation-independent assembly, said system isconfigured to select one or more different transmitted spectralpassbands at a time. This system may further comprise a passive linearlyvariable bandpass filter and/or a lens sub-system configured to directthe filtered light beam and couple it into an optical fibre or focus itonto an imaging or sensing device.

The polarisation-independent assembly of this system may comprise twopolarisers positioned either in a parallel or crossed configuration sothat the external field applied to each pixel or the group of pixelsfacing the beam coming from one single spectral passband is transmitted,while all other spectral bands are blocked. In a specific embodiment,this system comprises the three pixelated devices, wherein two of themform a two-stage Lyot filter, and the third device is installed betweensaid two polarisers positioned in the crossed configuration, said thirddevice is thus capable of blocking side interferences of the Lyotfilter, thereby improving the dynamic range of said tuneable filteringsystem by approximately the factor of two. Each one of the threepixilated devices in this system may be installed between said twopolarisers positioned in the crossed configuration.

In another embodiment, the tuneable filtering system, comprising atleast one pixelated device of the present embodiments, further comprisesa dispersive element producing an array of the filtered light beams.This dispersive element may be a prism or grating.

In still another embodiment, the tuneable filtering system, comprisingat least one pixelated device of the present embodiments, furthercomprises a passive wedged cavity, mirrors, lenses, and polarised beamsplitters, said system is configured to operate in a reflection modewithout the external field applied to said devices. The wedged cavitymay be empty or filled with any passive material.

A still another aspect of the present invention is a double-focusoptical system comprising the device of the invention, a birefringentplate, a lens, or a lens sub-system installed in the path of aconverging beam. An example of said lens sub-system is a microscopeobjective.

A further aspect of the present invention is anorthogonally-polarised-beam in-line interferometer for performingoptical measurements of an object comprising:

-   -   a) the double-focus optical system of the present embodiments;    -   b) an illumination unit directing the incident light beam onto        the microscope objective of the double-focus optical system;    -   c) a beam splitter installed in the path of the incident light        beam between the illumination unit and the microscope objective;    -   d) a phase-retardation modulator installed above the beam        splitter and configured to phase-modulate a combined beam;    -   e) a polarisation splitting mirror, which may be made of a        planar wire grid polariser and oriented with one side of said        polariser facing the object, wherein said mirror is capable of        reflecting only an ordinary wave component of the light beam and        installed after the device of the invention at the focal plane        of the ordinary wave component, thereby producing a reference        beam;    -   f) at least two non-polarising beam splitters configured to        split the combined beam into at least three beams directed        towards three detectors; and    -   g) at least three achromatic waveplates with their polarisers        installed in front of each of said detectors, thereby configured        to produce at least three different phase-shifted signals in        parallel;        wherein to preform optical measurements with said        interferometer, said object is placed at the focal plane of the        extraordinary wave component of the light beam.

Yet further aspect of the present invention is an orthogonally polarisedbeam in-line interferometer for operating with circularly polarisedbeams and for performing optical measurements of an object comprising:

-   -   a) the double-focus optical system of the present embodiments,        wherein the device of the invention further comprises a        birefringent helical structure capable of reflecting one        circular polarisation and transmitting the other;    -   b) an illumination unit directing the incident light beam onto        the microscope objective of the double-focus optical system;    -   c) a beam splitter installed in the path of the incident light        beam between the illumination unit and the microscope objective;    -   d) a phase-retardation modulator installed above the beam        splitter and configured to phase-modulate a combined beam;    -   e) a quarter waveplate installed between said helical structure        and the object, and within the temporal coherence region of the        light beam, thereby making both left and right circularly        polarised beams reflected from within the focal region of the        objective; and    -   f) at least two beam splitters configured to split the combined        beam into at least three beams directed towards three detectors,        wherein polarisers installed in front of each of said detectors        are oriented at different angles for providing at least three        different phase shifts between the two recombined circularly        polarised beams.

Another aspect of the present invention is a wide-range tuneablespectral filter comprising a coupling medium coated directly with amultilayer resonant structure, or connected optically with indexmatching fluid to a substrate coated with the resonant multilayerstructure, said multilayer structure reflects a narrowband tuned byvarying the angle of incidence or the refractive index or thickness ofone of the layers, wherein said resonant multilayer structurecomprising:

-   -   (a) A thin absorbing layer placed in contact with said coupling        medium;    -   (b) A low refractive index dielectric layer deposited on top of        said absorbing layer;    -   (c) The liquid crystal layer of the present invention        superimposed on top of said dielectric layer as a waveguide        layer;    -   (d) A top cladding layer with low refractive index to separate        the optical field from approaching the next electrode layer;    -   (e) A transparent top electrode layer coating the top panel of        said top cladding layer;    -   (f) A top substrate of low refractive index acting as        semi-infinite dielectric medium substrate on which the top        cladding layer and the transparent top electrode layer are        deposited; and    -   (g) Liquid crystal alignment layers deposited both on the bottom        and top cladding layers oriented so that maximum tunability is        obtained by applying electric or magnetic fields on the liquid        crystal.

In a particular embodiment, said one of the layers of the multilayerstructure acts as a waveguide. In another particular embodiment, saidthin absorbing layer placed in contact with said coupling medium isconducting. It is optionally made of Cr or W so it can act as a bottomelectrode in addition to functioning as the source for broadbandabsorption under total internal reflection conditions. In a specificembodiment, said top cladding layer with low refractive index and/orsaid top substrate of low refractive index is made of magnesium fluorideor silica oxide.

The coupling medium in this wide-range tuneable spectral filter may be aprism, gratings, a waveguide, or an optical fibre. In a specificembodiment, said coupling medium has a high refractive index, such as1.7 or higher. In a further specific embodiment, the absorbing layer isa conducting metal, such as chromium or tungsten, having thickness, forexample, 4-8 nm for the visible or near infrared range or thicker, suchas 15 nm for the short-wave infrared range. The exemplified lowrefractive index dielectric cladding layers are magnesium fluoride orsilica oxide having thickness, for example, in the range of 100-1500 nmdepending on the wavelength range used.

In a particular aspect of the present invention, a smart window or aprivacy window comprises the device of the invention. The smart orprivacy window may comprise the device of the invention, wherein itsmicroparticles are capable of strongly scattering the infrared radiationfrom the sun and partially reflecting or absorbing it, whiletransmitting the light in the visible range, thereby making said windowthermochromic smart and privacy window at the same time. Thesemicroparticles may be made of vanadium oxide,germanium-antimony-tellurium chalcogenide glass, zinc oxide, tungstenoxide or titanium oxide.

In a further embodiment, the smart window or privacy window comprisesthe device of the invention having the photosensitive layer, whereinsaid photosensitive layer is an intrinsic photo-conducting layer or aphotodiode layer, and the refractive index mismatch between saidmicroparticles and said liquid crystal is optically tuneable, saidwindow is designed to self-control its transparency depending on theincident light intensity. Bias voltages may be applied on thephotosensitive and liquid crystal layers to maximise the optical tuningeffect. Alternatively, if the optical tuning effect is large enough, thetwo transparent electrodes maybe shorted in order for the device toreturn to its original state when the tuning optical field is missing.The self-controlled transparency device maybe used as an energy savingprivacy window, protection window against laser or any UV radiation, oras eye goggles with self-controlled transparency.

The smart window or privacy window of the invention may further comprisean achromatic waveplate and a polariser, said waveplate is tuneable anddesigned to manipulate the polarisation state of the incident light andits transmission through said device, said device is in turn controlledvia said tuneable achromatic waveplate. Said polariser may be reflectiveor a metallic wire-grid polariser configured to reflect one polarisationcomponent of unpolarised light and transmit its other polarisationcomponent.

In yet further embodiment, the liquid crystal in the device of theinvention used in the smart window or privacy window may be a helicalanisotropic or helical photonic structure prepared by the oblique angledeposition technique, said liquid crystal is capable of reflecting onlycircular polarisation having the same helicity of the helical structureand transmitting the circular polarisation having the opposite helicity.This helical anisotropic liquid crystal may be chiral smectic,cholesteric or heliconical.

In another embodiment, the tuneable achromatic waveplate in the deviceof the invention used in the smart window or privacy window isconfigured to be switched-on, thereby acting as an achromatic quarterwaveplate, and thus, turning the light passing through it to circularlypolarised with the opposite helicity to the helix of said helicalstructure. In still another embodiment, said tuneable achromaticwaveplate is configured to be switched-off, thereby acting as a quarterwaveplate with 180-degrees phase retardation shift from the previousstate, and thus, turning the light passing through it to circularlypolarised with the same helicity as the helix of said helical structure.In an additional embodiment, said tuneable achromatic waveplate isconfigured to be switched at intermediate state, thereby providingelliptically polarized light, and thus controlling gradually the lightpassing through the window. In a particular embodiment, the pitch andbirefringence of the helical structure are selected to exhibit thecircular Bragg reflection phenomenon in the infrared range, and theachromatic waveplate is designed to operate in that same range.

A further aspect of the present invention is the device of the inventionused as a tuneable birefringent or tuneable phase plate, wherein saidtuneable birefringent or tuneable phase plate is a tuneable-focusachromatic liquid-crystal Fresnel diffractive-type lens made of annularregions. Each annulus in this lens is divided into several sub-annuli sothat different external fields are applied to each sub-region to causethe focal lengths of all wavelengths within wide spectral range tocoincide or to cause extension of the depth of field. This device canalso be used as a tuneable focus achromatic lens made by combining apassive diffractive Fresnel lens with a tuneable liquid crystal lens sothat the external fields applied to the liquid crystal lens causedispersion compensation and an achromatic combined focal length isachieved. The annular regions and sub-regions of the lens of thisembodiment may be replaced with rectangular regions and sub-regions sothat the tuneable achromatic lens becomes a tuneable achromaticcylindrical lens wherein two of such cylindrical lenses oriented at 90degrees to each other function as tuneable achromatic lens.

Various embodiments may allow various benefits and may be used inconjunction with various applications. The details of one or moreembodiments are set forth in the accompanying figures and thedescription below. Other features, objects and advantages of thedescribed techniques will be apparent from the description and drawingsand from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

Disclosed embodiments will be understood and appreciated more fully fromthe following detailed description taken in conjunction with theappended figures. The drawings included and described herein areschematic and are not limiting the scope of the disclosure. It is alsonoted that in the drawings, the size of some elements may be exaggeratedand, therefore, not drawn to scale for illustrative purposes. Thedimensions and the relative dimensions do not necessarily correspond toactual reductions to practice of the disclosure.

FIGS. 1 a-1 b schematically show the porous microparticles embedded inthe liquid crystal composite tuneable device of the present inventionfor polarisation-independent scattering.

-   -   FIG. 1 a illustrates a scattering state of the device in which        the microparticles infiltrated with the liquid crystal molecules        act as scattering centres due to large refractive index        mismatch.    -   FIG. 1 b illustrates a transparent state of the device in which        the liquid crystal molecules are oriented on the average along        one direction and hence, the refractive index mismatch is        minimized.

FIG. 2 illustrates a birefringence plate sandwiched between twopolarisers.

FIGS. 3 a-3 e schematically show the scattering medium which is achievedby having micro-domains at the panels randomly oriented.

-   -   FIG. 3 a illustrates random micro-fibres inside the liquid        crystal medium.    -   FIG. 3 b illustrates random micro-posts or micro-walls oriented        almost perpendicular between the two transparent panels, and the        liquid crystal molecules filling the space between them.    -   FIG. 3 c shows a skeleton structure of the micro-posts filled        with the liquid crystal.    -   FIG. 3 d shows a mesoporous matrix of random micro-fibres filled        with the liquid crystal.    -   FIG. 3 e illustrates an exemplary device of the embodiments with        one of the panels having different micro-domains (R1-R5) that        can induce different liquid crystal molecular orientations.

FIG. 4 a schematically shows the homogeneously aligned nematic liquidcrystal (LC) device comprising two transparent panels coated withelectrodes and alignment layers.

FIG. 4 b shows the geometry of the LC molecule, its principal refractiveindices and tilt angle.

FIG. 4 c schematically shows a modified configuration to obtain improvedgap uniformity by etching one of the transparent panels with the etchdepth equal to the desired cell gap.

FIG. 5 a schematically shows a polarisation liquid-crystal retardersystem with improved light throughput based on the Mach-Zehnderinterferometer configuration.

FIG. 5 b schematically shows the output of the LCD modulated beamreceived from the system shown in FIG. 5 a onto an etalon.

FIG. 5 c schematically shows the output of the LCD modulated beamreceived from the system shown in FIG. 5 a into an optical fibre and usethereof remotely as a light source.

FIG. 5 d schematically shows the output of the LCD modulated beamreceived from the system shown in FIG. 5 a and passed through a sampleonto collection or imaging optical system for measuring the spectralcharacteristics of the sample.

FIG. 6 schematically shows a liquid-crystal retarder system withimproved light throughput, based on the Mach-Zehnder interferometerconfiguration, having the bottom cell empty or filled and positioned at0 degree.

FIG. 7 schematically shows a liquid-crystal retarder system withimproved light throughput based on the Michelson interferometerconfiguration.

FIG. 8 a schematically shows a liquid-crystal retarder system withimproved light throughput based on the Sagnac interferometerconfiguration.

FIG. 8 b schematically shows a liquid-crystal retarder system withimproved light throughput based on the Sagnac interferometerconfiguration, having the bottom cell empty or filled and positioned at0 degree.

FIG. 8 c schematically shows a liquid-crystal retarder system with apolarisation conversion mirror, with improved light throughput, based onthe Sagnac interferometer configuration.

FIG. 8 d schematically shows a liquid-crystal retarder system with apolarisation conversion mirror, with improved light throughput, based onthe Sagnac interferometer configuration, having the bottom cell empty orfilled and positioned at 0 degree.

FIG. 9 a schematically shows a liquid-crystal retarder system withimproved light throughput, incorporating two Wollaston polarisers or twoRochon polarisers.

FIG. 9 b schematically shows a liquid-crystal retarder system withimproved light throughput, incorporating two Wollaston polarisers or twoRochon polarisers and having the bottom cell empty or filled andpositioned at 0 degree.

FIG. 9 c schematically shows a liquid-crystal retarder system withimproved light throughput, incorporating the Wollaston polariser or theRochon polariser, having the bottom cell empty or filled and positionedat 0 degree, and operating in a reflection mode.

FIG. 10 schematically shows a liquid-crystal retarder system withimproved light throughput based on the two-channel Mach-Zehnderinterferometer configuration.

FIG. 11 schematically shows a liquid-crystal retarder system based on athick liquid crystal Fabry-Perot interferometer.

FIG. 12 a schematically shows a liquid-crystal retarder system having aspatial separation configuration for converting one of the polarisationsinto the other.

FIG. 12 b schematically shows a liquid-crystal retarder system having anangular separation configuration for converting one of the polarisationsinto the other.

FIG. 13 schematically shows a liquid-crystal retarder system with apolarisation conversion mirror for polarisation-independent phase-onlymodulation.

FIG. 14 a schematically shows two anti-parallel aligned LCDs of thepresent embodiments positioned at 180 degrees to each other, so that theLC molecules of these two LCDs are mirrored to each other.

FIG. 14 b schematically shows the pi-cell configuration of the LCD ofthe present invention.

FIG. 15 a schematically shows the in-plane switching mode in which theoptic axis remains in the plane of the transparent panels of the LCD ofthe present invention.

FIG. 15 b schematically shows the ferroelectric mode of the LCD of thepresent invention.

FIGS. 16 a-16 b schematically shows the multiple domains modes of theLCD of the invention that create small neighbouring domains compensatingfor asymmetry in the angle dependence, thereby improving the field ofview.

FIG. 16 a shows a patterned vertically aligned mode of the LCD of thepresent invention.

FIG. 16 b shows a multi-domain vertically aligned mode of the LCD of thepresent invention.

FIG. 17 a schematically shows a wedge-type LCD of the presentembodiments with partially reflecting panels and producinginterference-coloured bands at its facet which can be selected seriallyusing the slit aperture.

FIG. 17 b schematically shows a spherical-type LCD of the presentembodiments with partially reflecting panels and producinginterference-coloured concentric rings at its facet which can beselected using the annular aperture.

FIG. 18 schematically shows a tuneable filtering system comprising theLCD of the present invention with a passive variable bandpass filter.

FIG. 19 schematically shows a pixelated liquid crystal compositetuneable device of the embodiments.

FIG. 20 schematically shows a tuneable filtering system comprising theLCD of the present invention with a dispersive element.

FIG. 21 schematically shows a double-focus system comprising the LCD ofthe present invention and a birefringent plate, a lens, or a lens systemin the path of a converging beam.

FIG. 22 schematically shows an orthogonally polarised beam in-lineinterferometer based on the system of FIG. 21 above.

FIG. 23 schematically shows an orthogonally polarised beam in-lineinterferometer with the cholesteric LCD of the present inventioncombined with a quarter waveplate.

FIG. 24 a schematically shows a wide-range tuneable spectral filtercomprising a tuneable waveguide layer such as the LC of the presentinvention and a resonantly reflective multi-layered guided wavestructure.

FIG. 24 b schematically shows a wide-range tuneable spectral filtercomprising the LC layer 100 of the present invention acting as thewaveguide layer within a resonantly reflective multi-layered guided wavestructure together with top cladding layer 22 b to prevent the guidedmode optical field from reaching the top electrode to prevent losses andthe two LC alignment layers 100 a.

FIG. 24 c schematically shows a prism made in the configuration allowingthe incoming incident beam and the output reflected beam to be in linewith each other, thus simplifying the incorporation of the tuneablefilter or modulator into optical systems.

FIG. 25 shows an exemplary simulated transverse electric reflectivityfrom the wide-range tuneable spectral filter.

FIGS. 26 a-26 d show simulations for a lower waveguide layer refractiveindex.

FIG. 27 shows a preliminary experimental result obtained usingmechanical scanning of a structure coated on SF11 substrate made ofthree layers: 6 nm Cr, 180 nm MgF₂, and 250 nm Ga₂O₃.

FIG. 28 schematically shows a smart or privacy window comprising apolariser, a tuneable achromatic waveplate and the LCD of the presentinvention.

FIGS. 29 a-29 b shows a prototype privacy window of the presentinvention with no voltage applied on the left and with applied voltageon the right.

FIG. 29 a shows the prototype privacy window based on the liquid crystalcomposite tuneable device of the present invention comprising the porousmicroparticles made of porous silica of about 2-3 μm in size and 4%concentration. The liquid crystal used in this example is Nematic BL036purchased from Merck, and the gap thickness was 12 μm.

FIG. 29 b shows the prototype privacy window based on the liquid crystalcomposite tuneable device of the present invention comprising thenon-porous microparticles made of non-porous silica of about 2-3 μm insize and 4% concentration. The liquid crystal used in this example isNematic BL036 purchased from Merck, and the gap thickness was 12 μm.

FIG. 30 a shows a parabolic profile of the optical path difference of arefractive lens.

FIG. 30 b shows the corresponding segmented profile of the refractiveindex of a diffractive lens (Fresnel lens).

FIG. 31 illustrates the method for manufacturing the liquid crystalcomposite tuneable device of the present invention.

DETAILED DESCRIPTION

In the following description, various aspects of the present applicationwill be described. For purposes of explanation, specific configurationsand details are set forth in order to provide a thorough understandingof the present application. However, it will also be apparent to oneskilled in the art that the present application may be practiced withoutthe specific details presented herein. Furthermore, well-known featuresmay be omitted or simplified in order not to obscure the presentapplication.

The term “comprising”, used in the claims, is “open ended” and means theelements recited, or their equivalent in structure or function, plus anyother element or elements which are not recited. It should not beinterpreted as being restricted to the means listed thereafter; it doesnot exclude other elements or steps. It needs to be interpreted asspecifying the presence of the stated features, integers, steps orcomponents as referred to, but does not preclude the presence oraddition of one or more other features, integers, steps or components,or groups thereof. Thus, the scope of the expression “a devicecomprising x and z” should not be limited to devices consisting only ofcomponents x and z. Also, the scope of the expression “a methodcomprising the steps x and z” should not be limited to methodsconsisting only of these steps.

Unless specifically stated, as used herein, the term “about” isunderstood as within a range of normal tolerance in the art, for examplewithin two standard deviations of the mean. In one embodiment, the term“about” means within 10% of the reported numerical value of the numberwith which it is being used, preferably within 5% of the reportednumerical value. For example, the term “about” can be immediatelyunderstood as within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%,0.1%, 0.05%, or 0.01% of the stated value. In other embodiments, theterm “about” can mean a higher tolerance of variation depending on forinstance the experimental technique used. Said variations of a specifiedvalue are understood by the skilled person and are within the context ofthe present invention. As an illustration, a numerical range of “about 1to about 5” should be interpreted to include not only the explicitlyrecited values of about 1 to about 5, but also include individual valuesand sub-ranges within the indicated range. Thus, included in thisnumerical range are individual values such as 2, 3, and 4 andsub-ranges, for example from 1-3, from 2-4, and from 3-5, as well as 1,2, 3, 4, 5, or 6, individually. This same principle applies to rangesreciting only one numerical value as a minimum or a maximum. Unlessotherwise clear from context, all numerical values provided herein aremodified by the term “about”. Other similar terms, such as “nearly”,“substantially”, “generally”, “up to” and the like are to be construedas modifying a term or value such that it is not an absolute. Such termswill be defined by the circumstances and the terms that they modify asthose terms are understood by those of skilled in the art. Thisincludes, at very least, the degree of expected experimental error,technical error and instrumental error for a given experiment, techniqueor an instrument used to measure a value.

As used herein, the term “and/or” includes any and all combinations ofone or more of the associated listed items. Unless otherwise defined,all terms (including technical and scientific terms) used herein havethe same meaning as commonly understood by one of ordinary skill in theart to which this invention belongs. It will be further understood thatterms, such as those defined in commonly used dictionaries, should beinterpreted as having a meaning that is consistent with their meaning inthe context of the specification and relevant art and should not beinterpreted in an idealized or overly formal sense unless expressly sodefined herein. Well-known functions or constructions may not bedescribed in detail for brevity and/or clarity.

It will be understood that when an element is referred to as being “on”,“attached to”, “connected to”, “coupled with”, “contacting”, etc.,another element, it can be directly on, attached to, connected to,coupled with, or contacting the other element or intervening elementsmay also be present. In contrast, when an element is referred to asbeing, for example, “directly on”, “directly attached to”, “directlyconnected to”, “directly coupled” with or “directly contacting” anotherelement, there are no intervening elements present. It will also beappreciated by those of skill in the art that references to a structureor feature that is disposed “adjacent” another feature may have portionsthat overlap or underlie the adjacent feature.

In one aspect of the present invention, a liquid crystal compositetuneable device (hereinafter, “liquid crystal device”, or “LCD” for thesake of simplicity) (100) for fast polarisation-independent modulationof a light beam comprises:

-   -   (a) at least two transparent panels, each coated with a        transparent conductive electrode layer and at least one of the        layers selected from alignment layer, antireflective coating        layer, thermochromic layer, or photosensitive layer, and    -   (b) a composite structure made of liquid crystal and porous        microparticles infiltrated with said liquid crystal, said        composite structure is sandwiched between said transparent        panels;        characterised in that:    -   (i) said porous microparticles infiltrated with said liquid        crystal have an average refractive index approximately equal to        one of the liquid crystal principal refractive indices;    -   (ii) an effective refractive index of the porous microparticles        matches that of the liquid crystal at one orientational state        (for example, parallel n₈₁) of said liquid crystal and exhibits        large mismatch at the other orientational state (for example,        perpendicular n_(⊥)), of said liquid crystal;    -   (iii) a refractive index mismatch between said microparticles        and said liquid crystal is tuned by applying an external        electric or magnetic field, thermally or optically; and    -   (iv) the microparticles concentration in said composite is less        than 50% for providing optimum contrast of the device upon        switching it between different orientational states of the        liquid crystal for privacy and smart window applications, and        preferably less than 0.1% for avoiding significant light        scattering on the microparticles for other photonic        applications.

In general, an index-matching material is a substance, such as liquid,gel or liquid crystal, which has a refraction index that closelyapproximates that of another substance, such as glass or polymer. Whenthe two substances with the same index are in contact, light passes fromone to the other with neither reflection, nor refraction or scattering.A mismatch between the refraction indices of the two substances inoptical devices usually causes serious problems, such as sphericalaberration, reduction of the effective numerical aperture by totalinternal reflection, and “fish tank” effect. However, the “refractiveindex mismatch” finds its application in the present invention as willbe discussed below.

The composite structure of the present embodiment, made of liquidcrystal and porous microparticles, is used in the LCD to modulate theincident light intensity with no polarisation dependence. Such devicecan therefore be used as a controlled transparency window. The ideabehind the invention is that the refractive index mismatch between themicroparticles and the liquid crystal, which surrounds and infiltratesthese microparticles, is tuneable upon applying external fields such aselectric or magnetic. The refractive index mismatch can also be tuneableunder thermal or optical impact.

By a “microparticle” we mean a particle having spatial dimensionstypically in the range of about 0.5-20 microns. The porousmicroparticles may be dielectric, magnetic, or metallic, preferably withhigh porosity, and may have a shape of tubes, rods, hollow fibres orshells. The aforementioned size of these microparticles is approximatelyin the order of magnitude or larger than the incident light wavelength.

The porous microparticles are advantageous over non-porousmicroparticles, since the surrounding liquid crystal molecules in thecomposite can infiltrate the pores of the microparticles and change therefractive index to achieve optimum modulation of the scattering.Another advantage of the porous microparticles is that their densitybecomes close to that of the LC as they get infiltrated with the LCmolecules and therefore sedimentation effects are minimized. Thetransparency of such device is thus polarisation-independent and can betuned with electric, magnetic, optical, or thermal fields. Infiltrationof the liquid crystal molecules into the porous microparticles impartsadditional functionality to the composite structure of the embodiments,depending on how the molecules are oriented inside the particle. Theaverage refractive index of the liquid crystal infiltrating the porousmicroparticle is close to the refractive index of the isotropic state ofthe liquid crystal:

n _(iso)=√{square root over ((2n _(⊥) ² +n _(∥) ²)/3)},

where n_(⊥) and n_(∥) are refractive indices of the liquid crystalmolecules perpendicular and parallel to (along) their long axisrespectively. The isotropic index is close to the refractive indexn_(⊥). As a result, for a homogeneously aligned liquid crystal device,the refractive index mismatch between the bulk of the liquid crystalmolecules and the microparticles is large, thereby resulting in largescattering. Therefore, and as mentioned above, the microparticles usedin the device of the present embodiments are preferably porous. Theseporous microparticles may be functional and made of thermochromicmaterial such as vanadium oxide, tungsten oxide, zinc oxide or titaniumoxide which allows their transparency to be controlled thermally orelectrically, or thermally and electrically. The microparticles may alsobe magnetic, photosensitive, fluorescent, or electrochromic.

Thus, the LCD of the present embodiments allows polarisation-independentintensity modulation and is based on tuneable scattering, which uses acomposite of a liquid crystal mixed with transparent microparticles.Reference is now made to FIG. 1 a-b showing schematically the porousmicroparticles embedded in the liquid crystal composite tuneable deviceof the embodiments for polarisation-independent scattering. Thesemicroparticles have a refractive index nearly equals to one of theliquid crystal refractive indices, for example n_(⊥), so that at zerovoltage, no index matching exists, and large scattering occurs. The LCDof the present embodiments comprises the transparent panels (1) coatedwith a transparent conductive electrode (TCE) layer and with anexemplary alignment layer, and the composite made of porousmicroparticles (2) surrounded by liquid crystal molecules (3). FIG. 1 aillustrates a scattering state of the LCD in which the microparticlesinfiltrated with the liquid crystal molecules act as scattering centresdue to large refractive index mismatch. FIG. 1 b illustrates atransparent state of the LCD in which the liquid crystal molecules areoriented on the average along one direction and hence, the refractiveindex mismatch is minimized. As the voltage increases, the liquidcrystal molecules re-orient with the field direction. As a result, theindex mismatch gradually decreases, and so does the scattering. Thecomposite structure of the embodiments may also be transparent at zerovoltage and scatters at high voltage, for example when the liquidcrystal material has a negative dielectric anisotropy andhomeotropically aligned.

The microparticles are formed by coagulation or agglomeration ofnanoscale particles so that the porosity of each coagulated particle ishigh. The porous microparticles may be prepared lithographically or bychemical etching, templating, electrospinning, embossing, laserpatterning or milling and then randomly distributed on at least one ofthe panels. Examples of such particles include porous silica, partiallyoxidised or nitrogenised porous silicon with various porosities andoxidation or nitrogenation levels. The microparticles may also beprepared at room temperature from a raw solid material, such as thesurfactant dimethyloctadecyl[3-(trimethoxysilyl)-propyl]ammoniumchloride (DMOAP) or N-methyl-3-aminopropyltrimethoxy-silane (MAP). Suchmaterials can be in solid form or in solution form.

This solid material preferably has its melting point slightly lower thanthat of the liquid crystal used in the composite. The composite isprepared by dissolving the raw material and mixing it with the liquidcrystal above the melting point, followed by cooling to roomtemperature, which then form microparticles embedded in the liquidcrystal matrix. Alternatively, both the liquid crystal and the rawmaterial of the microparticles may be dissolved in a common solvent suchas toluene and the composite is formed upon heating to the isotropictemperature, thus causing the solvent to evaporate and remaining withthe LC-microparticles composite. Since usually the surfactants cause theLC molecules to align perpendicular to the substrates plane, thenassuming the LC molecules have positive dielectric anisotropy, they willnot switch upon applying an electric field. In this case, it ispreferable to choose the liquid crystal having negative dielectricanisotropy which will then result in a composite exhibiting reversedbehaviour, that is transparent at zero voltage and scattering at highenough voltage. In a specific embodiment, the microparticlesconcentration in said composite is less than 50%, and preferably lessthan 30%, for providing optimum contrast of the device upon switching itbetween different orientational states of the liquid crystal for privacyand smart window applications. This concentration is furthermorepreferably less than 0.5%, and for other non-scattering photonic devicesapplications even more preferably less than 0.1%, for avoidingsignificant light scattering. The inclusion of less than 0.1%microparticles in the LC composite device of the present invention doesnot provide significant scattering but can improve the switching speedbetween different orientational states of the liquid crystal, because ofreducing the viscoelastic relaxation time. Similarly, for wavelengthsmuch larger than the microparticles size such as in the long waveinfrared or the THz ranges, the scattering becomes small andconsequently, the device may then be used as a tuneable birefringence orphase plate. The microparticles concentration is thus small enough tomake sure that the scattering is insignificant, and the device is usedfor phase, retardation or wavelength modulation, yet maintaining thefaster operation in any one of the configurations of the presentinvention.

Examples of the porous microparticles include glass, ceramic, metallic,ferromagnetic, photosensitive, or organic microspheres, which are mixedin a liquid crystal with 1-15% concentration. The microparticles mayalso be partially oxidized porous silicon microspheres or any otherporous dielectric microstructures. Oxidation level of the porousmaterial determines its adsorption degree, colour, refractive index, anda scattering degree of the composite. Therefore, tuneable architecturalprivacy windows, which are made from this material according to thepresent invention, exhibit different colours. Another preferableembodiment of the microparticles is to have their melting point lessthan that of the liquid crystal and maybe designed to fall near thesurrounding ambient temperature above which, the device will stop to bescattering. When the microparticles melt and get mixed with the liquidcrystal the scattering centres disappear and the liquid crystal layerbecomes transparent without dependence on the voltage. Hence with thisdesign one can get a thermochromic type device switching betweenscattering and non-scattering or vice versa depending on thetemperature.

As schematically shown in FIGS. 1 a -1 b, liquid crystal molecules arefrequently rod-shaped aligned, such that their long axes are on theaverage in the same direction making an angle θ, called the tilt angle,with the normal to the transparent panels. The panels (1) are usuallymade of glass treated at their facing surfaces with transparentconducting oxide (TCO) as electrodes and a thin polymer layer to providemono-domain alignment of the liquid crystal (LC) molecules. Some glassmicro-spheres (not shown in the figures) are inserted to act as spacersto determine the thickness of the device.

Because of their shape, the LC molecules are anisotropic and the wholedevice would therefore act as a birefringent plate. As noted above, themajority of the liquid crystal molecules are uniaxial meaning that theyhave two refractive indices, one along the molecule's axis (n_(∥)) andone perpendicular to it (n_(⊥)). When voltage is applied between the twoelectrodes connected to the transparent panels, the LC moleculesorientation changes. Thus, the tilt angle is a function of the voltage.As shown in FIG. 2 , when a linearly polarised light impinges on abirefringent medium, two waves can propagate at different velocities orrefractive indices, the ordinary and the extraordinary waves. Theextraordinary wave is an electromagnetic wave which is excited with itspolarisation along the projection of the molecular axis on thetransparent panels, and its index is a function of the tilt angle whichis a function of the applied voltage:

$n_{e} = \frac{n_{}n_{\bot}}{\sqrt{n_{\bot}^{2}} + {\left( {n_{}^{2} - n_{\bot}^{2}} \right)\cos^{2}\theta}}$

The ordinary wave index on the other hand remains fixed at n_(o)=n_(⊥),so that the effective birefringence defined as Δn=n_(e)−n_(o), variesfrom Δn=n_(∥)−n_(⊥) at zero voltage to near zero at high voltage whenthe molecules closely approach the orientation θ≈0 provided that themolecules have positive dielectric anisotropy. Because of the differencein their velocities the two excited waves (ordinary and extraordinary)accumulate a phase difference, Γ=2πdΔn/λ which depends on the pathlength they travelled d and birefringence. This phase difference Γcauses a variation in the polarisation state. As shown in FIG. 2 ,assuming that the polariser axis and the analyser axis are oriented atazimuthal angles P and A with respect to the optical axis (e-axis), thelight transmission can be written as:

T=cos² (P−A)−sin(2P)0×sin(2A)×sin²(Γ/2)

As an example, when liquid crystal is sandwiched between two crossedpolarizers and P=π/4, the transmission varies according toT=sin²(πdΔn/λ), while when the liquid crystal is between two parallelpolarizers, it is T=cos²(πdΔn/λ). This is the principle of polarisationor intensity modulation of the LCD of the embodiments. When the incidentlight polarisation is along the projection of the molecular axis on thetransparent panel plane, only one wave is excited. This wave is theextraordinary wave with its polarisation direction remaining unalteredhowever the device in this case acts as a variable index device. Thereare liquid crystal modes which by their nature exhibit isotropic changeof the refractive index such as blue phases of short pitch, and polymerdispersed liquid crystals with small LC droplets. Variable index modesare important to provide phase-only modulations which is of importancefor spatial light modulators.

However, because of the variation of the extraordinary index with thevoltage, its phase is varying. This is the principle of phase-onlymodulation action of the LCD of the embodiments. If the electrodes arepatterned into small pixels, then each pixel is able to modify a smallpart of an optical beam (polarisation, intensity, or phasemodification). As a result, a pixilated LCD can be used as a spatiallight modulator useful for improving image quality. Based on a spatialphase modulation, aberrations correction of wavefronts can be achieved.Similarly based on a spatial polarisation modulation, polarimetric orellipsometric images can be formed.

There are many LCD electro-optic modes which are based on the effectivebirefringence variation and the optic axis rotation when an electricfield is applied between the two electrodes. Non-limiting examplesinclude the hybrid mode, twisted nematic mode, in-plane switching mode,pi-cell, vertically aligned LC, smectic modes, chiral smectic mode,flexoelectric mode, smectic A, dual frequency mode, cholesteric LC withsmall pitch, blue phases, heliconical LC phases, distorted helixferroelectric (DHF), and surface stabilized ferroelectric LC (SSFLC). Inparticular, the smectic and cholesteric modes exhibit othercharacteristics when microparticles are embedded in them. Above certainconcentration, they break into regions forming focal conic texture,which is scattering. When a voltage is applied, they form a transparentnearly mono-domain state. Different voltage waveforms can be applied,and tuning can be achieved by varying the amplitude, the frequency orthe duty cycle of the voltage waveform. At sufficiently high frequencyand depending on the voltage applied, the temperature of the device mayincrease which changes the transparency of the device.

As it is evident from the above discussion, majority of the existingLCDs are usually polarisation-dependent, and therefore, the light beampassing through a linear polariser result in a 50% loss in the lightthroughput. This problem is thus solved in the present invention byintroducing a polarisation-independent device of the embodiments. Aswill be discussed below, privacy and smart windows are of many possibleapplications of this device. However, it can also be useful in manyother situations, for example when weak ambient light is used forimaging, or when two polarisers are used, for instance ininterferometric imaging and sensing applications. Non-limiting examplesare hyperspectral and wave-front correction devices based on liquidcrystals.

Another problem solved in the present invention is the strong dependenceon the angle of incidence. When a light beam impinges along the LCmolecules principal axis, the retardation vanishes, and phase modulationbecomes asymmetric with respect to the normal to the transparent panels.Because, in imaging systems, the angular extent of light contains a widespectrum of angles, each light beam may experience different phaseretardation. The devices of the embodiments are capable of extendingtheir angular field of view. Slow switching speed attributed to themajority of the existing LCDs, particularly when thick liquid crystallayers are used, is also overcome by the devices of the presentembodiments.

In a further embodiment, the composite structure may be formed fromliquid crystal by creating random liquid crystal micro-domains on atleast one of the panels, so that there is a large index mismatch betweenneighbouring micro-domains at one orientational state of the liquidcrystal. This mismatch actually decreases as the liquid crystalmolecular orientation changes with external field tuning. The alignmentlayer deposited on the transparent panels is made of random liquidcrystal micro-domains by one of the methods of nano-patterning of thetransparent conductive electrode (TCE) layer or photo-alignment of thephotosensitive polymer layer or thin chalcogenide glass. Thus, theliquid crystal micro-domains are created with refractive indexmismatching between them at one state (for example, parallel n_(∥)) andnearly full matching at the other state (for example, perpendicularn_(⊥)), which is tuned by an external field. In the above discussion,the index matching, or mismatching states (different orientationalstates of the liquid crystal) can be reversed depending on whether themicro-domains are hydrophilic or hydrophobic and whether the liquidcrystal has negative or positive dielectric anisotropy.

Reference is now made to FIGS. 3 a-3 e illustrating the above embodimentdescribing the scattering medium which is achieved by havingmicro-domains at the panels randomly oriented. As noted above, themicroparticles (2) may have a different shape, thereby resulting indifferent formed structures of the micro-domains. For example, FIG. 3 ashows random micro-fibres (2) inside the liquid crystal (3) medium. FIG.3 b shows random micro-posts or micro-walls (2) oriented almostperpendicular between the two transparent panels (1), and the liquidcrystal molecules (3) filling the space between them and maybe eveninfiltrating them at least partially. The micro-posts (2) may havevarious shapes, such as helices, cones, or zigzag shapes, which canreduce the angular dependence of the scattering. FIG. 3 c shows askeleton structure of the micro-posts filled with the liquid crystal.FIG. 3 d shows a mesoporous matrix of random micro-fibres filled withthe liquid crystal. FIG. 3 e illustrates an exemplary device of theembodiments with one of the panels having different micro-domains(R1-R5) that can induce different liquid crystal molecular orientationsso that the refractive index mismatch between the neighbouring domainsis large at one orientational state of the liquid crystal molecules andbecomes smaller at different orientational states.

The random orientation of the micro-domains causes large scattering atzero voltage. But when the voltage increases, the index mismatch betweenthe micro-domains starts to decrease and so does the scattering.Furthermore, the colour of the device may be tuned by selectingdifferent sizes of the microparticles, different raw materials,different porosities, or different refractive indices. As noted above,the porous silicon microparticles having different porosity andoxidation levels produce different colours of scattered light.Mesoporous silicon matrix at different oxidation levels is anotherpossibility for a random scattering structure which becomes tuneablewhen infiltrated with liquid crystals. In one of the embodiments, theporous microparticles may be functionalised with an organic or inorganicmolecular layer or nano-layer to control the way the liquid crystalmolecules are oriented on their surface and to maximise the randomnessof the micro-regions comprising the microparticles and the liquidcrystal surrounding. The pores size of the microparticles is not limitedand can range from few nm in diameter to hundreds of nm or even more.The larger the number of pores and their diameter, the better as long asthey are on the micron scale because then the effective index of theinfiltrated microparticle becomes closer to one of the principalrefractive indices of the liquid crystal and their density becomescloser to that of the liquid crystal thus minimising sedimentationeffects over time.

Organic or non-organic tubes normally used for drug delivery can be usedas the porous microparticles to produce the composite structure of theembodiments. One non-limiting example of such organic particles arecochleates which are cigar-like microstructures consisting of a seriesof lipid bilayers, formed as a result of the condensation of smallunilamellar negatively charged liposomes. In the presence of calcium,the small phosphatidylserine (PS) liposomes fuse and form large sheets.These sheets have hydrophobic surfaces and tend to roll-up into thecigar-like cochleate. Electron microscopy images usually show a typicalcochleate cylinder characterised by the elongated shape and by the tightpacked bilayers.

Cochleates are usually prepared by mixing DOPS (dioleoylphosphatidylserine) with DMPS (dimyristoyl phosphatidylserine) at the9:1 molar ratio, and then freeze dried to get the powder form. Thesamples are dialysed before freeze drying to remove salts. The obtaineddry powder can be directly used for analysis or by further preparing asuspension. The suspension samples are dried at room temperature undervacuum. The suspension of the cochleate particles should be sonicated totemporarily disrupt aggregates and avoid sedimentation of the particles.

Another example of the organic porous microstructure fibre maybe madefrom raw cotton, wastepaper or other organic materials. Such structurewhen embedded between the two transparent panels of the embodiments andfilled with the liquid crystal, forms the liquid crystal compositetuneable device of the present invention. In a further embodiment, themicro-particles are functionalised with a nano-layer of material thatcan cause preferable orientation of the liquid crystal molecules in thevicinity of the micro-particles. This will result in a randomly deformedstructure which reveals to higher switching contrast. An example of suchnano-layer coating is a surfactant, but any other functionalisationmaterial may be suitable.

In a particular embodiment, the microparticles or the mesoporousstructure shown in FIG. 3 d is made of thermochromic material, such asvanadium oxide, which is dielectric at room temperature, but becomesmetallic above its transition temperature. As a result, the deviceincorporating the vanadium oxide microparticles can in part stronglyscatter the infrared radiation from the sun in the summer period and inpart reflect or absorb it, while the transmission in the visible rangecan still be made high enough with proper selection of the voltageapplied to the device. Such device may act both as a privacy window andsmart window at the same time. The device may also be formed on a singlelayer of thermochromic material, such as vanadium oxide, a multi-layeredstructure or periodic structure containing vanadium oxide or anyelectrochromic material or phase change material, such as Ge₂Sb₂Te₅(GST) (chalcogenide glass), on one side of a window, while the compositeliquid crystal layer is adjacent to it. The thermochromic material thusacts as a smart window while the scattering composite liquid crystallayer acts as a privacy window.

The existence of the liquid crystal layer on top of the thermochromicmaterial helps in reducing reflections from the smart window structureboundary, thereby improving the transmission characteristics of thewindow. Birefringence of liquid crystals is reduced with the incidentlight wavelength. Therefore, at high temperatures, it is possible toadjust the voltage in order to maintain less reflection in the visiblerange of the spectrum while the reflection in the infra-red remainshigh. Thus, the liquid crystal layer in the device of the embodimentshas two roles: first, to provide the privacy window with itsfunctionality, and second, to actively control the Fresnel reflectionfrom the thermochromic material interface thus improving its contrast inthe infrared range and increasing the transmission in the visible range.

Thus, the randomly aligned micro-domains formed in the compositestructure of the embodiments result in strong scattering at zerovoltage. When voltage is applied, the liquid crystal molecules in allthe micro-domains reorient themselves with the electric field direction,thereby gradually decreasing the scattering as the voltage increases.The device of this embodiment is used in the thermochromic smart window,first, to turn the window to become both smart and privacy window, andsecond, to improve the transmission characteristics of the smart windowby modulation of the Fresnel reflection properties with the liquidcrystal composite tuneable device of the present invention. As mentionedabove, in all described scattering-mode devices, the index matched, ormismatched states can be reversed depending on whether the walls of theembedded micro-domains are hydrophilic or hydrophobic and whether the LChas negative or positive dielectric anisotropy. Bistability of thedevices can also be achieved depending on the choice of the liquidcrystal, for example ferroelectric LC are known to exhibit bistabilityas well as metal-organic liquid crystalline compounds.

The LCD of the embodiments may also be tuned optically by depositing aphotosensitive layer, such as an intrinsic photo-conducting layer orphotodiode structure, on at least one of the transparent panels. Atleast part of the solar spectrum impinging on the photo-sensitive layerside changes the voltage drop across the liquid crystal layer, therebymodulating the scattering. Some photo-conducting layers, such aschalcogenide layers, can be made thin to transmit large part of theincident light. They can also act as photo-alignment layers. Ofparticular interest is a photosensitive layer capable of absorbing theUV part of the solar spectrum and converting it into voltage drop acrossthe device. Thus, in addition to the efficient energy consumption, suchlayer also provides protection from the UV radiation. Example of suchhighly efficient UV photoconductive layer is an aluminium-dopedzinc-oxide nanorod array annealed in oxygen environment.

Similarly, photo-conducting polymers can be used as the photosensitivelayers for visible light, while InGaAs can be used as a photoconductivelayer in the short-wave infrared range (SWIR). In yet furtherembodiment, a single layer is deposited on one of the transparent panelsor two layers are deposited on the two panels on top of the transparentconductive electrode layer. Other examples of more transparentphotovoltaic device include Cu based chalcogenide glasses, mixed withperovskites, and using highly transparent electrodes such as metalgrids. Although the voltage change produced by solar light or part of itmight not be enough to switch the liquid layer completely, it is stillpossible to apply a bias voltage to maximise the effect of sunlight.This optically addressed device can be used as a smart window capable ofself-controlling its transparency depending on the sunlight intensity.In the summer period, the solar intensity is strong, so this window mayautomatically dim and keep the house cold, while in the winter period,when there is no strong sunlight, the window may brighten, therebykeeping the house warm.

Thus, the use of a photoconductive or photovoltaic layer in conjunctionwith the composite structure allows modulating the smart window'stransparency by the photo-voltage or resistance change due to theincident sunlight shining on the window.

Reference is now made to FIG. 4 a schematically showing a typicalbirefringent device, which in the present exemplary case is a nematicliquid crystal device having an antiparallel geometry. The geometry ofthe dielectric tensor associated with the liquid crystal molecules isshown in FIG. 4 b . The polariser and analyser are installed to obtainintensity- or wavelength-modulation. For phase-only modulation, onlypolariser is installed, while for polarisation-independent operation,neither polariser, nor analyser is required.

One of the difficulties in liquid crystal devices used in photonicapplications is obtaining uniform thickness. As shown in FIG. 4 c , animproved gap uniformity can be obtained by etching one of the glasssubstrates with the etch depth equals to the desired cell gap. The panelsurfaces are then coated with the necessary layers of transparentconducting oxide and alignment materials, and with optional layers, suchas dielectric mirrors for a Fabry-Perot cavity tuneable filter orphotoconductive layers. The second (bottom) panel is not etched, butsimply pressed and glued to the bottom of the device without spaces.

To obtain easy electrical connections the substrates are laterallyshifted by an offset so that the connection area is exposed on bothsubstrates from right and from left. Using modern etching techniques(chemical, ion-beam, or laser-beam) one can achieve nanoscale uniformityover large area. Although not shown in these figures, different meansfor easy electrical connections are possible, for example by drillingholes in the transparent panels followed by filling them with aconducting material. This is important for pixelated devices in whichthe number of electrical connections increases with the number ofpixels.

According to another aspect of the present invention, variousconfigurations of the LCD of the present embodiments having a tuneablebirefringence are described below. All these configurations based on theLCD of the invention represent tuneable, polarisation-independentoptical systems effectively overcome the aforementioned problems of theexisting LC devices.

System Configuration 1: Mach-Zehnder Interferometer

In one embodiment, a polarisation liquid-crystal retarder system withimproved light throughput comprising the LCD of the present invention isshown in FIG. 5 a . It is based on the Mach-Zehnder interferometerconfiguration having an arbitrarily polarised beam split with aconventional polarised beam splitter (4) into two orthogonally polarisedand collimated beams P and S. The conventional Mach—Zehnderinterferometer determines the relative phase shift variations betweenthese two collimated beams derived by splitting light from a singlesource. The generated phase shifts between the two beams P and S iscaused by the sample or a change in length of one of the paths.

As shown in FIG. 5 a , the P-beam passes through the liquid crystaldevice (100) of the embodiments oriented at 90 degrees, meaning that itsoptic axis is parallel to (or along) the P-polarisation direction.Hence, this beam is affected by the extraordinary refractive index thatvaries with the applied voltage. The S-beam passes through anothersection of the LCD (100) and is affected by the ordinary refractiveindex, which does not vary with the applied voltage.

Upon recombining the P- and S-beams, the resulting combined beam isagain arbitrarily polarised, but modulated as the voltage varies due tothe resulting phase retardation Γ=2πd(n_(e)−n_(o))/λ. This is similar tothe situation when the incident light beam is linearly polarised at 45degrees to the optics axis. However, in the present system, there is noloss in the light that is usually encountered when the input linearpolariser is used.

Reference is now made to FIGS. 5 b, 5 c and 5 d schematically showingthree uses of the output modulated beam from FIG. 5 a . In FIG. 5 b ,the modulated beam is passing through an optical etalon having an outputtransfer function of multiple narrow spectral bands, so that bymodulating the LCD (100), the output spectrum includes multiple narrowspectral bands variable with the voltage. In FIG. 5 c , the modulatedbeam is passing through an output of an illuminator connected to anoptical fibre which can be used remotely as a light source with variousspectroscopic or imaging systems. In FIG. 5 d , the modulated beam beingreflected or transmitted through a sample is then directed to a paralleldetector or to an imaging system either for spectral measurementapplications, or for hyperspectral imaging purposes, respectively. Inorder to obtain the polarisation-independent phase modulation in theconfiguration of FIG. 5 a , a half-waveplate or polarisation rotatorshould be inserted in the S-beam path both before and after the cell, sothat the beam passing the cell becomes P-polarised, and then convertedback to S to be reflected by the polarised beam splitter (4).

Reference is now made to FIG. 6 showing the similar configuration asabove, but with the bottom cell (5) empty (without liquid crystalinside) or with the filled cell oriented at 0 degree. The S-beam is nowpassing through either the empty bottom cell or a filled cell orientedat 0 degree (5). The filled cell must be oriented at zero degree so thatboth the P-and S-polarisation have the same phase modulation. When thebottom cell (5) is filled, this configuration can work without theoutput polariser and for the phase-only polarisation-independentmodulation, which is useful in tuneable lensing and wavefrontmodulation. Similarly, to the previous configuration, thepolarisation-independent phase modulation can also be achieved in thisconfiguration, provided that, for example, a half-waveplate orpolarisation rotator is inserted in the path of the S-beam both beforeand after the cell, so that the light beam passing the cell (5) becomesP-polarised and converted back to S to be reflected by the polarisedbeam splitter (4).

In case of the empty cell, the phase retardation is Γ=2πd(n_(e)−1)/λwhich is larger by ΔΓ=2πd(n_(o)−1)/λ from the previous configurationshown in the FIG. 5 a . Taking typical values for the refractive indicesn_(e)=1.78 and n_(o)=1.52, the calculated phase retardation willincrease by a factor of 3-4, which allows thinning of the liquid crystallayer by the same factor. Since switching time of nematic liquidcrystals is proportional to the square value of the thickness, theresponse time of the device may be improved by a factor of 9-16. This isa significant improvement for spectral modulation devices withcompressed sensing, FTIR spectroscopy, or phase modulators operated in along wavelengths range (IR or THz). Thus, using the presentconfiguration, the system of the present embodiments can beminiaturised, but still benefit from the short switching time reducedfrom seconds to milliseconds or less.

System Configuration 2: Michelson Interferometer

Reference is now made to FIG. 7 illustrating the system of theembodiments based on the Michelson interferometer that uses polarisationconversion mirrors in both arms for light throughput and speedimprovement. The whole device maybe a single solid unit with the armsglued (using the refractive index matching glue) to the sides of thepolarised beam splitter (4).

The conventional Michelson interferometer uses a beam splitter splittinga light beam into two arms. Each of those light beams is reflected backtoward a beam splitter which then combines their amplitudes using thesuperposition principle. The resulting interference pattern that is notdirected back toward the source is typically directed to some type ofphotoelectric detector or camera.

Similarly, in the present configuration, the S-polarised beam is firsttransmitted through either an empty cell (5) or a filled cell positionedat 0 degree, reflected from a first polarisation conversion mirror (6′)converting the beam to P-polarisation, and then transmitted through asecond polarisation conversion mirror (6″). The polarised P-beam is thendirected to the filled LCD (100) oriented at 90 degrees, reflected fromthe second polarisation conversion mirror (6″), then converted toS-polarisation and again reflected from the first polarisationconversion mirror (6′) to recombine with the beam from the otherinterferometer channel.

The polarisation conversion can be performed by several means, forexample using a quarter waveplate (QWP) and regular mirror, a Faradaymirror, a metallic grating with the Gaussian profile of the grating'slines or other means. The net phase retardation in this geometry for thecase of empty cell in the bottom arm is Γ=2πd(n_(e)+n_(o)−2)/λ. However,since it is not variable with the voltage, the same modulationcharacteristics can be achieved as in the previous case but with a fixedphase shift. The cell (5) in the bottom arm can be empty to enhance theretardation modulation or filled, oriented at 0 degree to providepolarisation-independent phase modulation. In the latter case, theoutput polariser is not necessary.

System Configuration 3: Sagnac Interferometer

A Sagnac interferometer (or a ring interferometer) is based on aphenomenon of interference that is elicited by rotation. A beam ofincident light is split and the two beams are made to follow the samepath but in opposite directions. On return to the point of entry, thetwo beams are allowed to exit the ring and undergo interference. Therelative phases of the two exiting beams, and thus the position of theinterference fringes, are shifted according to the angular velocity ofthe apparatus. Thus, when the interferometer is at rest with respect tothe earth, the light travels at a constant speed. However, when theinterferometer system is spun, one beam of light will slow with respectto the other beam of light. Fibre-optic and ring-laser gyroscopes arebased on this phenomenon.

Reference is now made to FIG. 8 a showing a polarisation liquid-crystalretarder system with improved light throughput based on the Sagnacinterferometer configuration. In this configuration, the P and S-beamspass twice through the LCD (100) with the help of a retroreflector (7)or two mirrors. The phase retardation is doubled, Γ=4πd(n_(e)−n_(o))/λ,with the advantage of having the two beams passed along the same path.

In another embodiment, the configuration shown in FIG. 8 b is similar tothe previous configuration shown in FIG. 8 a but has an empty cell (5)(without liquid crystal). In this case, the phase retardation becomesΓ=2πd(n_(e)−n_(o))/λ, which is twice less than in the previous case(with liquid crystal). As mentioned above, the empty cell (5) can befilled with the liquid crystal, but then it should be oriented at 0degree, and no output polariser is needed. In this configuration,polarisation-independent phase-only modulation is achieved, which isimportant for many applications including polarisation-independentvirtual reality applications and tuneable focusing.

In still another embodiment, the Sagnac interferometer configurations,which are shown in FIGS. 8 a -8 b, may be further supplemented with apolarisation conversion mirror (6) installed instead of theretroreflector (7). The respective Sagnac interferometer configurationsare shown in FIG. 8 c -8 d. In all these configurations shown in FIGS. 8a -8 d, the retroreflector (7) or the polarisation conversion mirror (6)may constitute one of the transparent panels (1) of the LCD (100) of theembodiments or attached with refractive index-matching glue to one ofthe external sides of the transparent panels (1).

System Configuration 4: Wollaston Polariser, Rochon Polariser

A Wollaston polariser consists of two birefringent right-angle triangleprisms cemented together, such that their optical axes areperpendicular. It separates randomly polarised or unpolarised light intotwo orthogonal linearly polarised outgoing beams. As light passesthrough such polariser, a symmetric deviation between the ordinary andextraordinary beams is created. The resulting beams are of orthogonallinear polarisation states and have equal intensity and a large angulardeviation, which is determined by the prisms' wedge angle and thewavelength of the light. Commercial Wollaston polarisers are availablewith divergence angles from 15° to about 45°.

A Rochon polariser is very similar to the Wollaston polariser, but theordinary beam passes through the prism without deviation. The Rochonpolariser also consists of two birefringent material prisms in opticalcontact with one another. As the ordinary beam is not deviated on bothsides of the prism, the ordinary and extraordinary beams remaincollinear through the first prism. Upon entering the second prism, theordinary rays do not experience a change in the refractive index andpass through the prism without deviation, while the extraordinary raysrefract at the interface.

Reference is now made to FIG. 9 a showing a polarisation liquid-crystalretarder system of the embodiments incorporating either two Wollastonpolarisers or two Rochon polarisers (8′ and 8″). As mentioned above,these two polarisers have the property of splitting the incident beaminto two orthogonally polarised beams with a small angle in between. Thetwo P- and S-beams are then collimated using a lens, passes the liquidcrystal device (100) of the present invention, collected again andrecombined by a polariser at the output.

The similar configuration, but with the bottom cell (5) empty (withoutliquid crystal) or filled with the liquid crystal and positioned at 0degree is shown in FIG. 9 b . In this case, one beam passes through thefilled LCD (100) and the other beam passes through the empty bottom cell(5) or filled and oriented at 0 degrees. The phase retardation in thecase with the filled bottom cell (5) is Γ=2πd(n_(e)−n_(o))/λ, while inthe case with the empty bottom cell (5) is Γ=2πd(n_(e)−1)/λ, which islarger. Therefore, the latter configuration shown in FIG. 9 b with theempty bottom cell (5) is preferable. When the bottom cell is filled,polarisation independent modulation is achieved and no need for theoutput polariser.

Reference is now made to FIG. 9 c showing the equivalent configurationwith only one Wollaston polariser or with one Rochon polariser (8) butoperating in a reflection mode with the advantage of doubling the phaseretardation to Γ=4πd(n_(e)−1)/λ. The polarisation-independent phasemodulation can also be achieved in all the configurations shown in FIGS.9 a -9 c, provided that, for example, a half-waveplate or polarisationrotator is inserted in the path of the S-beam both before and after thecell, so that the light beam passing the cell (5) becomes P-polarisedand converted back to S to be recombined with the other beam in theWollaston or Rochon polariser.

System Configuration 5: Two-Channel Mach-Zehnder Interferometer

Reference is now made to FIG. 10 showing the Mach-Zehnder interferometerconfiguration with two channels. It is similar to the configurationshown in FIG. 5 a except that there are two LCDs (100′ and 100″), andtheir optic axes are aligned at 45 degrees with respect to the P or Spolarisations, both at the top and at the bottom of the system. As aresult, the two beams become modulated, and when passing through theoutput polarised beam splitter (4), two modulated channels of the sameor differently modulated beams are obtained. This configuration has noloss in light throughput and the advantage of having two modulated beamsat the same time, which can be useful for measuring two differentsamples simultaneously.

System Configuration 6: Fabry-Perot Interferometer

A Fabry—Pérot interferometer (resonator or etalon) is a linear opticalresonator (or cavity) which consists of two highly reflecting parallelmirrors or made of a transparent plate with two reflecting surfaces(having some small transmissivity) and is often used as ahigh-resolution optical spectrometer.

Reference is now made to FIG. 11 schematically showing a systemconfiguration comprising the LCD of the present invention and based on athick liquid crystal Fabry-Perot interferometer, which produces largenumber of narrow spectral bands in the transmission spectrum. The narrowbands shown in FIG. 11 can be modulated with the applied voltage. Thedashed peaks in the spectrum are obtained at voltage V1, while the solidcurves are obtained at voltage V2. The tuneable multiple-bandFabry-Perot can also be used in multispectral imaging with the bandsselected are the ones of interest, thus replacing the passive multiplebandpass filter. This configuration has an advantage of achieving highresolution within shorter processing time in the compressed sensing orcomputational spectroscopy methodologies.

The system configuration shown in FIG. 11 may be combined with colouredparallel detectors. As an example, consider an output of threewavelengths centred at the standard colour camera band at 450 nm, 550and 650 nm. Since the three bands are tuned, three spectral images areobtained at each voltage, and the final number of spectral imagesobtained is the number of the voltages multiplied by 3. Similarly, withthe relatively modern multispectral parallel detectors, it is possibleto design the Fabry-Pérot resonator with the LCD of the presentinvention to produce peaks centred at the major wavelengths of thecoloured pixels, and by tuning them the larger number of spectral imagescan be obtained. The thick LCD (100) of the present invention shown inFIG. 11 is sandwiched between either two regular dielectric mirrors orpolarisation conversion mirrors (6). In the case of the polarisationconversion mirrors, the output is polarisation-independent, therebycreating larger light throughput. In case of the regular mirrors, thelight has to be linearly polarised.

In yet further embodiment, the dielectric mirrors (6) in FIG. 11 may bemade in a way that the reflection and transmission through the deviceexhibit interference with no need for polarisers, for example by havingdifferent reflectivity at different wavelengths. The tuneablebirefringent element (TBE) sandwiched between these two mirrors can thentune these colours by applying an external field. If the TBE layer ismade thick enough, then the colours can disappear at the highretardation state, because the different interference orders becomeclose to each other. As the retardation decreases, for example byapplying voltage to the LCD (100), the interference orders becomeseparated, and colours start to appear depending on the applied voltageor temperature. Thus, using this configuration, an interference-basedsmart window can be built.

The system configurations of the above embodiments, which incorporateboth the LCD (100) and the empty cell (5) can be used for measurementsof a refractive index or thickness of transparent materials. Theseconfigurations acting as orthogonal polarisation interferometers havethe LCD sample arm (100) with the S-polarisation and the empty cellreference arm (5) with the P-polarised beam. As described above, thereference arm may contain the cell filled with liquid crystal ortuneable birefringent element instead and then should be positioned at 0degree. The output phase of the light traversing the sample can bemeasured similar to the phase-shift interferometry by providingdifferent known phase changes to the P-polarised beam passing thetuneable birefringent element. The output analyser is a must in thiscase in order to combine the two beams. It is then possible to extractthe phase change of the light traversing the sample from the outputsignal after the output analyser by providing at least three knowndifferent phase differences between the reference and sample arms. Thiscan be used for refractive index measurement of gases and liquidsflowing through the empty cell or for solid transparent materials. Inthe case of the solid transparent materials, it is possible to measuretheir thickness, assuming that the refractive index is known.Alternatively, it is also possible to extract intrinsic birefringence inthe sample and calculate stresses in the sample material, for example,in glass.

In a further embodiment, the system of the present invention can beconfigured to convert one of the polarisations into the other. Thesystem shown in FIG. 12 a has a spatial separation configuration, whilethe system shown in FIG. 12 b has an angular separation configuration.The polarisation conversion in FIG. 12 a is done with the help of thepolarised beam splitters (4) and polarisation conversion mirrors (6).The LCD (100) must be oriented at 45 degrees to the output polarisationdirection, and the net phase retardation is then calculated asΓ=2πd(n_(e)−n_(o))/λ with the advantage of having higher lightthroughput.

The configuration shown in FIG. 12 b has a further advantage of usingthe flat elements, such as a wire grid polariser (9) or an achromaticquarter waveplate (not shown). The wire grid polariser (9) lets theP-polarisation to pass but reflects the S polarisation. Consequently,the S-polarised beam upon reflection from the polarisation conversionmirror (6) becomes P-polarised, thus passing through the wire gridpolariser (9) at different angle from the initial P-polarised beam. Theangles of the two beams should be small in order to avoid any modulationvariations over different sections of the beams. The liquid crystaldevice (100) positioned at 45 degrees, capable of minimising the angledependence, such as pi-cell, is preferably used in this case.

Reference is now made to FIG. 13 showing a polarisation liquid-crystalretarder system with a polarisation conversion mirror (6) forpolarisation-independent phase-only modulation. In this configuration,the polarisation conversion mirror (6) is installed behind the LCD (100)oriented at an arbitrary angle. In a specific embodiment, thepolarisation conversion mirror (6) may comprise a quarter waveplate(QWP) combined with a mirror, or it can be made of a metallic gratingwith the grating lines having Gaussian profile. In another specificembodiment, a Faraday rotator can also operate as the polarisationconversion mirror (6) in the reflection mode.

The P polarisation is converted to S upon reflection, while S convertsto P. As a result, the two polarisations accumulate the same phasemodulation as shown in FIG. 13 . In fact, the retarder does not have tobe oriented at a specific angle for the polarisation-independentphase-only modulation to occur. For the retarder with its optic axisoriented at an angle ζ, the Jones matrix maybe written as:

${W = {{\exp\left( {- i\Gamma_{av}} \right)}\begin{pmatrix}{{\cos\left( {\Gamma/2} \right)} - {i\cos 2{{\xi sin}\left( {\Gamma/2} \right)}}} & {- i\sin 2\xi{\sin\left( {\Gamma/2} \right)}} \\{- i\sin 2\xi{\sin\left( {\Gamma/2} \right)}} & {{\cos\left( {\Gamma/2} \right)} + {i\cos 2\xi{\sin\left( {\Gamma/2} \right)}}}\end{pmatrix}}},$

where Γ_(av)=2πd(n_(o)−n_(e))/2λ is the average retardation. Inreflection using a polarisation reflection mirror, the Jones matrixbecomes:

$W_{tot} = {{i{\exp\left( {- 2i\Gamma_{av}} \right)}\begin{pmatrix}{{\cos\left( {\Gamma/2} \right)} - {i\cos 2{{\xi sin}\left( {\Gamma/2} \right)}}} & {i\sin 2{{\xi sin}\left( {\Gamma/2} \right)}} \\{i\sin 2\xi{\sin\left( {\Gamma/2} \right)}} & {{\cos\left( {\Gamma/2} \right)} + {i\cos 2\xi{\sin\left( {\Gamma/2} \right)}}}\end{pmatrix}\begin{pmatrix}0 & 1 \\1 & 0\end{pmatrix}\begin{pmatrix}{{\cos\left( {\Gamma/2} \right)} - {i\cos 2{{\xi sin}\left( {\Gamma/2} \right)}}} & {- i\sin 2{{\xi sin}\left( {\Gamma/2} \right)}} \\{- i\sin 2\xi{\sin\left( {\Gamma/2} \right)}} & {{\cos\left( {\Gamma/2} \right)} + {i\cos 2\xi{\sin\left( {\Gamma/2} \right)}}}\end{pmatrix}} = {{i{\exp\left( {- 2i\Gamma_{av}} \right)}\begin{pmatrix}{{\cos\left( {\Gamma/2} \right)} - {i\cos 2{{\xi sin}\left( {\Gamma/2} \right)}}} & {i\sin 2{{\xi sin}\left( {\Gamma/2} \right)}} \\{i\sin 2\xi{\sin\left( {\Gamma/2} \right)}} & {{\cos\left( {\Gamma/2} \right)} + {i\cos 2\xi{\sin\left( {\Gamma/2} \right)}}}\end{pmatrix}\begin{pmatrix}{- i\sin 2{{\xi sin}\left( {\Gamma/2} \right)}} & {{\cos\left( {\Gamma/2} \right)} + {i\cos 2\xi{\sin\left( {\Gamma/2} \right)}}} \\{{\cos\left( {\Gamma/2} \right)} - {i\cos 2\xi{\sin\left( {\Gamma/2} \right)}}} & {- i\sin 2{{\xi sin}\left( {\Gamma/2} \right)}}\end{pmatrix}} = \text{ }{i{\exp\left( {- 2i\Gamma_{av}} \right)}\begin{pmatrix}0 & 1 \\1 & 0\end{pmatrix}}}}$

As a result, the assembly of an arbitrarily oriented retarder andpolarisation conversion mirror acts as a polarisation-independentphase-only modulator with the phase modulation equal to 2Γ_(av). Thebeam splitter (4′) shown in FIG. 13 is optional and may be removed ifthe light incidence is at oblique angle. Alternatively, the Sagnacconfigurations shown in FIGS. 8 a-8 d can be used in this system. Insome of the LCD modes, such as the ferroelectric LC modes, twisted modesor in-plane switching modes, the optic axis rotates in the plane of thetransparent panels and is characterised by the angle ζ. Hence thepresented polarisation-independent phase-modulation configuration canalso work with ferroelectric liquid crystals which are faster.

In a further embodiment, the spectral- or phase-modulated LCDs of thepresent invention are used in imaging systems which usually containlarge angular extent. It is therefore critical to minimise the angulardependence of the phase retardations. Reference is now made to FIGS. 14a-14 b showing the LCDs of the present invention that are capable ofminimising the angle dependence. For example, FIG. 14 a shows twoanti-parallel aligned devices of the present invention positioned at 180degrees to each other, so that the liquid crystal molecules of the topLCD mirror the molecules of the bottom LCD. FIG. 14 b shows theconfiguration with the parallel aligned LCD, i.e., pi-cell. Since thetwo halves of the pi-cell are mirror images of each other, this devicehas wider field of view than the anti-parallel aligned LCD. Theexemplary liquid crystal modes in which the optic axis remains in theplane of the LCD transparent panels are the in-plane switching modeshown in FIG. 15 a and the ferroelectric mode shown in FIG. 15 b .Another exemplary liquid crystal mode is shown in FIGS. 16 a-16 billustrating the LCD with compensating waveplates and other modescontaining multiple domains: (a) a patterned vertically aligned mode;and (b) a multi-domain vertically aligned mode, in two voltage regimes(switched of and switched on).

The LCD of the present invention can be modified to exhibit colouredbands on its surface which can be tuned with the voltage. Reference isnow made to FIG. 17 a showing a wedge-type LCD of the presentembodiments with partially reflecting panels (10). Each panel (10) iscoated with a transparent electrode and alignment layer. This LCD has athickness varying in the range between d₁ and d₂ over the length L ofthe retarder, so that the wedge angle is determined by tan ω=(d₂−d₁)/L.Because of the wedge, the coloured interference bands appear on thefacets of the wedge which can be selected serially using the slitaperture. These coloured bands correspond to the maxima in transmissionor reflection. Similarly, as shown in FIG. 17 b , the wedge can bespherical, thereby producing the spectral (coloured) concentric ringsappearing on the surface. These coloured rings (spherical bands) can beselected using the annular aperture. In both cases, the same effect canbe obtained using a flat uniform LCD with highly resistive electrodes athigh-frequency operation, so the voltage across the electrode isnon-uniform and the colours will therefore appear due to the voltagedistribution. The voltage non-uniformity in this case is a function ofthe frequency and electrical resistance. The highly resistive electrodeshave conducting contacts on one side for activating the linear wedge andan annular contact around the liquid crystal active area for activatingthe spherical wedge. In the case of the linear wedge, the beam-steeringeffect can be made tuneable with the voltage because of the linearrefractive index gradient. In case of the spherical wedge, the tuneablelensing effect can be created by applying symmetric voltage profile whenusing the annular highly conducting electrodes around the liquid crystalactive area. This tuneable lensing effect makes it possible to use apolarisation-independent tuneable lens in the configurations of theembodiments described above for polarisation-independent phase-onlymodulation.

Thus, there are two configurations which can impart colour variationsacross the liquid crystal composite tuneable device of the presentinvention:

-   -   1) The LCD of the present invention has a negligible reflection        from internal surfaces of its two transparent panels. In this        case, the device should be oriented between crossed or parallel        polarisers at the 45-degrees azimuthal angle, and the colour        bands appear at the maxima of the transmission functions T=sin²        (πdΔn/λ) or T=cos²(πdΔn/λ) depending on the product dΔn which        varies because of the wedge (d-variation) or the voltage (Δn        variation).    -   2) The LCD of the present invention has a high reflection from        internal surfaces of its two transparent panels. In this case,        multiple interferences having maxima at the surface of the        device take place, when the condition for constructive        interference on the surface of the device is fulfilled,        depending on dn_(o,e)/λ which varies because of the wedge        (d-variation) or the voltage n_(e)-variation) or the wavelength        (λ).

The interference pictures shown in FIGS. 17 a-17 b suggest that it ispossible to place an aperture (rectangular slit or circular aperture)centred at d_(c) and having a width Δ, so that only one colour passes ata particular voltage. If the transmission function is T(λ, d), the totaltransmission of the spectrum from λ₁ till λ₂ of the slit is given by:

${T_{slit} = {\int\limits_{\lambda_{1}}^{\lambda_{2}}{\int\limits_{d_{c} - {\Delta/2}}^{d_{c} + {\Delta/2}}{{T\left( {\lambda,t} \right)}d\lambda{dt}}}}},$

where the variable d is changed to t inside the integral. Thus, thenarrower the aperture is used, the narrower the spectral line isselected.

Reference is now made to FIG. 18 showing a tuneable filtering systemcomprising the LCD (100) of the present invention with a passivelinearly variable bandpass filter (11), which is commercially available,for example from Delta Optical Thin Film, and prepared on a transparentsubstrate. The common mode of using such filters is by mechanicallymoving them. However, mechanical motion is slow and might add noise tothe imaging or sensing system. In the system configuration of thepresent embodiment, the LCD (100) is combined with the linearly variablepassive filter (11), and therefore, it is capable of selecting one ormore different transmitted spectral passbands at a time. A lens system(12) then directs the filtered beam and couples it into an optical fibre(13) or focuses it onto an imaging or sensing system (not shown in thefigure).

As shown in FIG. 19 , the LCD of the present invention may be pixelated.Such pixelated LCD (100) is integrated within a polarisation-independentassembly (14) which is shown in FIG. 18 . The pixelated LCD (100)further comprises two polarisers (not shown here) positioned either in aparallel or crossed configuration, so that the voltage on each pixel orgroup of pixels facing the beam coming from one single passband istransmitted, while all other bands are blocked.

A single LCD (100) is capable of selecting a limited range ofwavelengths and relatively wide band beam. However, for a wide range andnarrow-band operation, three LCDs maybe installed in the system. Two ofthem form a two-stage Lyot filter, and the third is installed betweenthe crossed polarisers, so it can block the side interference peaks ofthe Lyot filter, thereby improving the dynamic range by approximatelythe factor of 2. Another option would be to use three LCDs, each one ofthem installed between the crossed polarisers and having their LC layerthicknesses as d, 3d and 5d, thereby providing a wider dynamic range andcreating the narrowband beams with a smaller full width at the halfmaximum (FWHM), compared to a single LCD system.

FIG. 20 schematically shows a tuneable filtering system comprising theLCD of the present invention with a dispersive element (15) havingappropriate optics. The dispersive element (15) can be a prism orgrating and can produce the array of filtered beams. The system of thepresent embodiment can operate also in a reflection mode withappropriate and commercially available mirrors, lenses, and polarisedbeam splitters arrangements. In this configuration the linearly variablepassive filter (11) of the previous configuration may also be replacedwith a passive wedged cavity similar to that shown in FIGS. 17 a -17 b,but without the voltage applied to the liquid crystal layer.Alternatively, the linearly variable passive filter (11) may be replacedwith an empty cavity or a cavity filled with any passive material. Incase of the spherical cavity, the pixelated LC device of FIGS. 18 and 20should select an annular zone which is variable with the voltageapplied.

Reference is now made to FIG. 21 schematically showing a double-focussystem comprising the LCD (100) of the present invention and abirefringent plate, a lens or a lens system (16) installed in the pathof a converging beam. It is known from ray optics that if a glass blockof thickness d_(g) is located in the path of a focused beam, the focalpoint shifts by an amount ΔF_(g)=d_(g)(tan α_(g)/tan α_(i)−1), whereα_(i), α_(g) are the incidence angle on the glass and the refractionangle inside the glass. These two angles are connected via Snell's lawstating that sinα_(i)=n_(g) sin α_(g), assuming that the glass block islocated in air. However, the glass is usually isotropic, and thereforethe two polarisations P and S are focused at the same point. In thepresence of the LC layer having the thickness d_(LC), these twoorthogonal polarisations experience two different refractive indicesn_(o) and n_(e) inside the LC layer (“o” stands for “ordinary” and “e”stands for “extraordinary”. As a result, the two polarisations arefocused at focal planes separated by:

${{\Delta F_{LC}} = {\frac{d_{LC}}{\tan\alpha_{i}}\left( {{\tan\alpha_{e}} - {\tan\alpha_{o}}} \right)}},$

which for small angles can be approximated asΔF_(LC)≈d_(LC)(n_(e)−n_(o))/n_(e)n_(o).

Since the refractive index of the extraordinary ray in nematic LCs canbe varied between n_(⊥) and n_(∥), a double focus imaging is obtained attwo orthogonal polarisations with the distance between the two focalplanes variable by the amount:

ΔF _(LC) ≈d _(LC)(n_(∥)−n_(⊥)/) n _(∥) n _(⊥)

Taking the typical values of the LC refractive indices of n_(∥)=1.76 andn_(⊥)=1.51, and thickness d_(LC) of 100 μm, the calculated ΔF_(LC) willbe approximately 9.8 μm. As a result, using the thicker LC layer and amaterial with higher birefringence, it is possible to obtain highertunability of the focus. Thus, using this configuration severalfunctionalities can be achieved:

-   -   1. If the light is polarized and the LCD is oriented such that        only the extraordinary ray is excited, a tuneable focus is        achieved using a planar LCD of the present invention. This can        be used to perform the Z-scan in confocal microscopy or optical        coherence tomography. By dynamically changing the focus, an        extended depth of field imaging can be obtained. The same can be        obtained using the polarisation-independent LCD of the        embodiments, for example using the configurations of the        embodiments described above with the advantage of no need for        polarising the incident light.    -   2. Using the LCD of the present invention, which is tuneable        faster than the camera frame rate, the focal plane may be        modulated by dynamically tuning the LCD, so that the final image        grabbed by the camera will have an extended depth of field.        Although the contrast of the image will deteriorate in this        case, some minor image processing can bring it back to nearly        the same quality of a focused image.    -   3. A double focusing for each polarisation with one of the focal        planes (extraordinary wave) can be made tuneable for unpolarised        light, elliptically polarised light or linearly polarised light        containing both P and S components, so that both the ordinary        and the extraordinary beams are excited inside the birefringent        LC layer. This functionality can be used for orthogonal        polarisation interferometry or interference microscopy, as will        be detailed with reference to FIG. 22 below.

It is possible to use a single channel (one camera or detector), butproduce the phase shift between the two beams using a phase modulatorlocated above the beam splitter as shown in FIG. 21 . Reference is nowmade to FIG. 22 schematically showing an orthogonally polarised beamin-line interferometer based on the system configuration shown in FIG.21 . A polarisation splitting mirror (17) reflecting only the ordinarywave is introduced at the focal plane F_(o) (ordinary), therebyproducing a reference beam, while the object of interest is located atthe focal plane F_(e) (extraordinary). The polarisation splitting mirror(17) may be made of a planar wire grid polariser or using themulti-layered flat polarizing beam splitter available from 3Mindustries. If the path length difference between the two beams iswithin the coherence region of the incident beam, the almost common-pathorthogonal polarisation interferometry is obtained. In thisconfiguration, the two orthogonally polarised beams are split into threechannels, and a waveplate is inserted in each channel to producedifferent phase shifts. The two orthogonal polarisations are thenrecombined at an analyser plane located before each detector (camera orsingle detector). Without the objective and projection lenses, thisinterferometer can still operate as an in-line parallel-beaminterferometer (an orthogonal-polarisation interferometer with parallelbeams) and may be used for serial phase modulation. There are manyapplications of this interferometer in the field (where sub-nmresolution range and the axial direction matter), for example surfacetopography, focus tracking and vibrometry.

Reference is now made to FIG. 23 schematically showing an orthogonallypolarised beam in-line interferometer having the cholesteric LCD (100)of the present invention combined with a quarter waveplate (19). Thequarter waveplate (19) is inserted near the object of interest withinthe focal region of a microscope objective (18) and within the temporalcoherence region of the beam, so that both left and right circularlypolarised beams are reflected from within the focal region of theobjective (18). The reflected wave from the cholesteric LCD (100) iscircularly polarised having the same helicity as the cholesteric LChelix, while the transmitted wave has the opposite helicity. Thisselective reflection phenomenon is also known as the circular Braggreflection phenomenon. The opposite helicity wave passes through thequarter waveplate (19), get reflected from the object and passes againthrough the quarter waveplate (19). Upon this round trip the wave'shelicity remains unchanged, and therefore, it consequently passes backthrough the cholesteric LCD (100). As a result, there are two beams thatare orthogonally polarised (right and left circularly polarised) whichgive the rise to the in-line orthogonally polarised interferometer ofthe present embodiments.

The two beams are brought together to interfere on a detector. At leastthree phase shifts can be introduced between the two beams seriallyusing a modulator, or in parallel using at least three channels havingdifferent phase shifts. Since the two interfering beams are nowcircularly polarised, the phase shifts can be introduced by passing thetwo beams through the linear polarisers positioned at differentorientations without the need for waveplates. The serial phasemodulation can be achieved by rotating the linear polariser. Since theLCD (100) reflects one circular polarisation at certain range ofwavelengths, the operating wavelengths can be chosen to be within thereflection band of the LCD (100).

It is possible to tune the reflection band, and consequently, to tunethe operation spectral band by applying voltage to the LCD (100). Thisis really important if the multiple-wavelengths phase-shiftinterferometry mode is used which allows overcoming the phase unwrappingproblems without any complicated algorithms. This interferometer canstill operate without the objective and projection lenses, simply as anin-line orthogonal-polarisation interferometer. Another group of helicalstructures exhibiting the circular Bragg phenomenon can be equally usedinstead of the cholesteric LCD such as the chiral sculptured thin filmsprepared by the glancing angle deposition technique.

The double focus for two orthogonally linearly polarised beams can alsobe achieved using an LC lens or a combination of the LC phase-onlyspatial light modulator with a standard lens. To achieve this, aparabolic radial mask is written on the spatial light modulator planewith large number of pixels. The ordinary wave is not affected this way.It is focused in the close proximity to the original focal plane of thelens. The extraordinary wave is to the contrary modulated and shifted bythe amount determined by the maximum phase written on the spatial lightmodulator and on the numerical aperture of the lens. A rough estimatefor the case when the distance between the spatial light modulator andthe lens is very small results in Δf=λ∅_(max)/πNA².Polarisation-independent tuneable focusing can then be obtained bycascading two devices oriented at 90 degrees to each other, or bycombining the proposed device with the polarisation-independent assemblyconfiguration described above.

In another embodiment, the spatial light modulator may be used to extendthe depth of field of the imaging system by placing it in the exit pupilplane and writing the annular regions on it that provide equal phaseshifts, but variable from one annulus to another. In a specificembodiment, the spatial light modulator uses several parabolic profilesof the phase, each corresponding to slightly different focal point. Thecombination of the several focus regions provides an extended depth offield. Alternatively, one can scan the different phase shift masks fastenough, so that an average image is obtained with the extended depth offield. Minor image processing can then bring the image back to nearlythe same quality as the original focused image.

In yet further embodiment shown in FIG. 24 a , a wide-range tuneablespectral filter comprises a coupling medium (20), such as a prism,gratings, a waveguide or an optical fibre, coated with a multilayerstructure comprising a thin absorbing layer (21), such as metal (forexample, chromium metal of 4-8-nm thickness for the visible and nearinfrared range, or thicker for larger wavelengths) in contact with saidcoupling medium (20), a low refractive index dielectric layer (22)acting as first clad or coupling layer (for example, magnesium fluorideor Si oxide having few hundreds of nanometre thickness) on top of saidabsorbing layer (21), the liquid crystal composite tuneable device (LCD)(100) of the present invention superimposed on top of said dielectriclayer (22), a transparent electrode layer (23) coating the toptransparent panel (1) of said LCD (100), and a semi-infinite dielectricmedium layer (24) on top of said electrode layer (23). The absorbinglayer (21) preferably be conducting such as absorbing metal, semimetal,or semiconductor to act also as a first electrode. This structure isactually a backward resonating structure with the resonance tuned by theliquid crystal composite tuneable device of the present invention. Thetunability is obtained by modulating the optical path (thickness timesthe refractive index) of the waveguide layer (100). Hence in principleany transparent thermochromic, electrooptic, magnetooptic, piezoelectricor photosensitive layer can be used. The electrodes are required whentuning is to be done using electric field, however for magneto-optic,optical or thermo-optic tuning no electrodes are required. The resonancemaybe interpreted in several terms, such as a special type of a guidedmode resonance, Fano resonance or coupled waveguides resonance.Tunability can be achieved by modulating an external magnetic field,electric field, optical field, or thermal field applied to the filter.Since for this device, the modulation required is phase-only, then theLCD layer (100) maybe even replaced with other electro-optic,magneto-optic, photosensitive or thermo-optic material, which mayprovide easier means of preparation or operation or maybe exhibitingultrafast tuning of the resonance wavelength. The dielectric layer (22)thickness strongly affects the full width at half maximum, and it ispossible to get extremely narrow peaks by increasing the thickness ofthis layer. This is not possible by either using plasmonic structuresbecause of their high absorption, or by the standard guided moderesonant structure at a wide spectral range as demonstrated here. TheLCD layer with the two bounding alignment layers 100 a is shown in FIG.24 b . To minimize losses due to interaction between the optical fieldin the waveguide and the top electrode, a low refractive index layeracting as a top clad layer maybe added between the waveguide layer andthe top electrode layer. Note that a metallic absorbing layer 21 can beused as the bottom electrode in addition to functioning as the source ofbroadband lossy surface wave. The prism may also be made in theconfiguration shown in FIG. 24 c to allow the incoming incident beam andthe output reflected beam to be in line with each other thus simplifyingthe incorporation of the tuneable filter or modulator into opticalsystems.

The incident light beam can be generated after polarisation splittingusing one of the system configurations of the embodiments describedabove. Thus, the entire system based on this backward resonatingstructure further comprises polarisation conversion elements to get thetwo polarisation components modulated similarly by the same device. Thetuneable spectral filter of the present embodiment can be combined withthe system configurations described above to achievepolarisation-independent operation.

The absorbing layer (21) and its combination with the dielectric layer(22) allows propagation of several types of surface electromagneticwaves, such as Zennick wave, Tamm wave, or surface plasmon resonancewave, however the choice of highly absorbing layer to excite a broadbandlossy wave is the best as it allows the wide tuning range of theresulting Fano resonance. The LCD (100) of the present inventionincorporated in this tuneable spectral filter is thick enough to allowguided waves. The interaction between the two different types of waves(surface electromagnetic waves and guided waves) causes a resonance inreflection due to constructive interference in the backward direction.The location of this resulting resonance is highly sensitive to theliquid crystal properties and therefore, can be used as a tuneablefilter or a refractive index sensor. The advantages of this tuneablefilter are in its wide range tuning, narrowband and fast responsebecause the thickness of the LCD layer can be thinner than 1 micron forvisible range operation. As mentioned above, other electro-optic,magneto-optic, photosensitive or thermo-optic materials can be usedinstead of the LCD layer.

As an example, FIG. 25 shows a simulated transverse electricreflectivity from the wide-range tuneable spectral filter of the aboveembodiment shown in FIG. 24 . In this example, the coupling medium (20)is a right angle SF11 glass prism, the angle of incidence inside theprism is 52 degrees, and the stack of layers is 6-nm Cr (21)/575-nm MgF₂(22)/603-nm liquid crystal (100) oriented so that the extraordinary modeis excited, and the refractive index is variable as shown near eachresonance peak/MgF₂ layer (22).

Considering the fact that the speed of electrooptic materials usuallyincreases as their electrooptic effect becomes weaker, it is of highimportance to increase the sensitivity of the resonance to the waveguidelayer refractive index. This can be achieved by choosing the refractiveindex of the waveguide layer closer to that of the cladding. Todemonstrate this, FIGS. 26 a-26 d show simulations for lower waveguidelayer refractive index. The conditions are similar to the case in FIG.25 except that the liquid crystal (100) layer has thickness of 1000 nmin FIGS. 26 a and 26 c , while it is 2000 nm in FIGS. 26 b and 26 d .Typically the refractive index difference between the clad and waveguidelayers should be less than 0.1 and the waveguide layer thickness shouldbe at least few wavelengths.

Note that for the transverse magnetic (TM) wave a dip is obtained inreflectivity and not a peak. This is observed in the majority of casesdepending on the phases accumulated by this wave, however there arecases when a peak is observed also for TM wave, although not at the sameposition as that for the TE wave and it comes broader in width.Therefore, for polarization independent operation it is important tocombine this structure with the different configurations proposed inthis invention particularly those which convert the TM into TEpolarization so that narrower filter is obtained.

In FIGS. 26 a and 26 c , it is clearly seen that the resonance shifts bymore than 500 nm upon changing the WG refractive index by 0.06, almostthree times higher sensitivity than the case of FIG. 25 , meaning thesensitivity increased by around factor of x3-x4. The sensitivity hasstrong dependence on the different parameters of the structure andoptimum conditions can be easily found as the structure contains mainlyplanar stratified layers which can be modelled using Abeles 2×2 matrixmethod easily and with the LC layer incorporated possible also with the4x4 matrix approach.

When the spectral sensitivity increases, the angular one increasesdrastically showing that in this case tuning within more than 300 nmrange occurs within less than a degree change of the incidence angle.Two important phenomena occur as the difference between the refractiveindex of the waveguide (WG) n_(w) and that of the cladding n_(c),δn=n_(w)−n_(c) decreases:

-   -   (i) the WG thickness has to increase in order to bring the peak        back to the VIS-NIR range, which is easily understood from the        WG mode equation;    -   (ii) splitting of the peak into two with increasing gap as δn        increases similar to Rabi type splitting known in other systems        such as when coupling occurs between extended and localized        plasmons.

The new emerging peaks (FIG. 26 b ) shift in opposite manner to theoriginal ones, meaning they blue shift as the WG refractive indexincreases and as the WG thickness increases or the incidence angledecreases. Even when δn=n_(w)−n_(c) is not so large the splitting isover more than 500 nm, so one can use the newly emerging peak tunabilityfor practical applications over broadband.

FIGS. 26 c and 26(d) show for the case of WG thickness d_(w)=2000 nm andrefractive index n_(w)=1.417−1.384 tunability over 700 nm range. Sincethe required refractive index of the electrooptic layer (LC or others)and its modulation needed are both small, this opens new possibility forultrahigh speed modulators and tuneable filters, even with liquidcrystals. Hence this structure, being planar so it is easy to fabricateit in large area and being rich in applicable phenomena certainlytriggers further industrial applications.

In yet another embodiment it is possible not to have the first claddingor what we called coupling layer. This requires the WG layer thicknessto be larger and then we cannot control the full width at half maximum(FWHM) of the reflected peak.

The FWHM can be controlled by changing the thickness of the couplinglayer, the larger it is the shorter the FWHM. Therefore, although thisembodiment is easier to fabricate it is not the most preferrable. Formore detailed analysis on the different variations of the structure thereader is referred to the published article by the inventor (IbrahimAbdulhalim, Tuneable filter and modulator with controlled bandwidth andwide dynamic range based on planar thin films structure, Optics Express27(11), 16156-167 (2019)).

Another important embodiment of the resonant structure is as anoptically addressed spatial light modulator (OASLM) by having either:

-   -   (i) with the waveguide layer being made of photosensitive        material to one wavelength range not experiencing the resonant        reflection to modulate the resonantly reflected waves of        different spectral range where the photosensitive layer is        transparent, or    -   (ii) with the waveguide layer being adjacent to a layer        photosensitive to one wavelength range not experiencing the        resonant reflection to modulate the resonantly reflected waves        of different spectral range where the photosensitive layer is        transparent.

Another important application of the invented resonance structure is forgenerating large number of reflected peaks within the same spectralrange. This can be done simply by increasing the thickness of thewaveguides, as the number of modes that can be excited in the waveguideis proportional to the thickness. In this sense the structure willprovide multiple spectral peaks similar to what a Fabry-Perot cavity orpassive bandpass filter can provide. Using the LC layer or any otherelectrooptic material one can then tune the array of spectral peaks,hence a fast spectral modulator can be built, useful for hyperspectralor multispectral imaging when combined with computational spectroscopytechniques.

Another important embodiment of the invention is the use of thestructure as a high-resolution dispersive element due to the strongangular sensitivity of the reflected peak to the incidence angle. Usinga collimating beam with angular scanning it is possible to get then amechanically tuneable filter. Because the scanning required is as smallas one degree, this can be a fast mechanically tuned filter. FIG. 27shows a preliminary experimental result obtained using mechanicalscanning of a structure coated on SF11 substrate made of three layers: 6nm Cr, 180 nm MgF₂, 250 nm Ga₂O₃. The SF11 substrate was attached toSF11 right angle prism using index matching oil. Although the scanningwas done manually and therefore the experimental error is relativelyhigh (estimated +/−10%) the general behaviour observed is as expectedfrom the simulations, thus proving the main concept. For a demonstrationwith LC layer, the proposed technique of glass etching is used in orderto obtain high uniformity of the LC layer thickness. Another embodimentthat uses the high dispersion property of the structure is to use a onedimensionally diverging beam diverging in the plane of incidence, whichthen upon reflection will generate a rainbow of colours or wavelengthscorresponding to the resonant peaks as function of angle. The wavelengthdistribution can then be measured in one shot using a detectors array orcamera, hence acting as a spectrometer. Alternatively. a scanned slitcan be used at the output to generate the different filtered beams thusacting as a monochromator.

As mentioned above, the liquid crystal composite tuneable device of thepresent invention can be used in privacy or smart windows. The systemcomprising the LCD (100) of the present invention for use in suchtuneable windows is illustrated in FIG. 28 and further comprises apolariser and an achromatic waveplate (25). The achromatic waveplate(25) is tuneable and designed to manipulate the polarisation state ofthe incident light and its transmission through the LCD (100), which isin turn controlled via the tuneable achromatic waveplate (25). Helicalanisotropic LCDs, such as chiral smectic LCDs, cholesteric LCDs,heliconical LCDs, or helical photonic crystals prepared by the obliqueangle deposition technique, are examples of the LCDs of the presentinvention used in the privacy or smart windows of the embodiments. Thesehelical structures reflect only one circular polarisation which has thesame helicity of the helical structure. The circular polarisation havingthe opposite helicity is transmitted through.

The reflection band centre wavelength is determined by λ_(p)=Pn_(av)with P being the period of the helix and n_(av) is the averagerefractive index. The full width at half the maximum of the reflectedpeak is given by FWHM=PΔn, where Δn is the local effectivebirefringence. Therefore, by choosing the helix pitch in the centre ofthe visible spectrum, i.e., P=550 nm, with the high-birefringencematerial having Δn=0.25, the obtained reflection band covers most of thevisible range from blue to red. It is also possible to have the pitch inthe infrared range to manipulate the infrared part of the solarspectrum.

The polariser shown in FIG. 28 is preferably reflective, such as ametallic wire-grid polariser reflecting one component of the unpolarisedsolar spectrum, while the other component is transmitted. Nearly 50%reflection from the polariser surface prevents seeing clearly from oneside of the privacy window, whereas the other side of the window iscompletely transparent for seeing due to the light part that transmittedthrough. The linear polarisation is transmitted through the tuneableachromatic waveplate (25) which can be at several switching states:

-   -   1) In the “Switch-ON” state, it acts as an achromatic quarter        waveplate, so that the light passing it becomes circularly        polarised with the opposite helicity to the helix of the helical        LCD. The light is then transmitted through the helical LCD        giving a total transmission close to 50%.    -   2) In the “Switch-OFF” state, it acts as a quarter waveplate        with 180-degrees phase retardation shift from the previous state        so that the light passing it becomes circularly polarised with        the same helicity as the helix and thus reflected with the same        helicity. Because the reflected light is at the same helicity,        it will be converted to the same linear polarisation        (transverse-magnetic, in case of the wire grid) when it passes        back through the quarter waveplate and also through the        polariser. As a result, the window becomes totally dark from one        side and completely reflective, like a mirror, from another        side.    -   3) At any intermediate state, the transmitted light through the        polariser becomes mostly elliptically polarised upon passing        through the tuneable achromatic waveplate, and the intermediate        transmission state can be controlled with an external field        (voltage, thermal, optical, magnetic). At these intermediate        states, some light reflection from the helical LCD may leak back        through the polariser, thus making the window looks coloured,        which is an important property from an architectural point of        view.

FIGS. 29 a-29 b shows a prototype privacy window of the presentinvention with no voltage applied on the left and with applied voltageon the right. FIG. 29 a shows this privacy window based on the LCD (100)of the present invention comprising the porous microparticles made ofporous silica of about 2-3 μm in size and 4% concentration. The liquidcrystal used in this example is Nematic BL036 purchased from Merck, andthe gap thickness was 12 μm. FIG. 29 b further shows this window basedon the LCD of the present invention comprising non-porous silicamicrospheres of 2.54 μm and 4% concentration. The liquid crystal used inthis example was also Nematic BL036 purchased from Merck, and the gapthickness was 12 μm. These two figures compare the use of the porousversus non-porous microparticles in the device of the present inventionand clearly demonstrate the superiority of using the porousmicroparticles over non-porous microparticles.

The tuneable window described herein may also be used to transmit 50% ofthe visible light of the solar spectrum at all times while controllingthe transmission of the infrared light. This can be accomplished bychoosing the pitch of the helix to be in the microns range and theachromatic waveplate to operate in the infrared range. This way thewindow acts as a smart window to keep the house cool during the summerperiod and warm during the winter period.

As described above, two LCDs of the present invention, having theiroptic axis preferably oriented at 90 degrees with respect to each other,may be combined in one system. In that case, the incident polarisationis at 45 degrees with respect to the optic axis of the first LCD in thesystem. Such system, when driven at different voltages applied to thetwo different LCDs, provides excellent tunability between an achromatichalf wave, a quarter waveplate and a full waveplate.

In another embodiment, an achromatic tuneable lens comprises the LCD ofthe present invention. In yet further embodiment, an imaging systemcomprises an achromatic tuneable lens, which is a refractive lens, and adiffractive lens, for example the Fresnel-type lens having a negativedispersion. The focal length of this diffractive lens is dependent onthe wavelength of the incident light, so that the longer the wavelengthis, the shorter the focal length is, which is opposite to the focallength dispersion of a refractive lens.

The action of an element as a lens requires lateral variation of theeffective refractive index n_(eff). The optical path length differenceexists between the rays passing through the centre of the lens and atsome distance r from the centre:

OPD=2πd(n _(c) −n _(eff)(r))/λ

The parabolic profile is usually the desired one to minimiseaberrations, so that the transmission function of the lens is describedby:

${{t(r)} = {\exp\left\lbrack {\left( {- {jkd}} \right)\left( \frac{n_{c} - {n_{eff}(r)}}{R^{2}} \right)r^{2}} \right\rbrack}},$

where R is the radius of the lens, and d is its thickness. The focallength of such refractive lens is given by:

${f = {\frac{R^{2}}{\left. {2{d\left( {n_{c} - n_{eff}} \right)}\left( {r = R} \right)} \right)} = \frac{R^{2}}{2{d\left( {n_{c} - n_{p}} \right)}}}},$

where n_(c) is the refractive index at the centre of the lens (for apositive lens, n_(c)−n_(p)>0).

In the LCD of the present invention, or any LC lens, the focal lengthcan be tuned because of the control of n_(e)(r) with an external field.FIG. 30 a shows a parabolic profile of the optical path difference(wavelength) of a refractive lens with the M maximum. Since thedispersion of glasses and LC materials is such that the refractive indexdecreases as the wavelength increases, it is evident from the aboveequation of the focal length f that the focal length increases as thewavelength increases. In other words, the blue colour is focused first,then the green colour and then the red colour. This phenomenon isusually observed with the human eye, which is a typical refractive lens.

Reference is now made to FIG. 30 b showing the corresponding segmentedprofile of the refractive index represented in a wrapped way of adiffractive Fresnel lens. In this presentation, the lens radius isdivided into five regions and the phase is wrapped five times so thatthe maximum optical path difference at each zone is M/5 waves. However,to achieve the same total optical path difference of M waves, the zonesbecome denser as the distance from the centre increases. In the Fresnellens configuration, the radius of zone j is given by r_(j)≈√{square rootover (jλ₀f)}, where λ₀ is the design wavelength. Because of thisspecific design for the specific wavelength, the focal length varieswith the wavelength in an opposite way to the refractive lens. Morespecifically it varies according to

${f(\lambda)} = {\frac{\lambda_{0}}{\lambda}{{f\left( \lambda_{0} \right)}.}}$

Therefore, the red colour is now focused first, then the green and thenthe blue, opposite to the dispersion of the refractive lens. However,the focal length at the design wavelength has the dispersion due to therefractive index dispersion according to the above equation of the focallength f . This means that under assumption the diffractive lens zonesare made from a material having the standard refractive index dispersion(index decreases with the wavelength), some compensation may occur, andthe final focal length of the Fresnel lens might not have significantdispersion. However, this is not enough in many cases where theachromatic operation is required, for example in imaging systems.

According to the present embodiment, to get the achromatic operation ofthe imaging system, the diffractive lens is combined with the refractivelens having exactly the same focal length dispersion in magnitude, butopposite in sign. If the two lenses are attached together, then thetotal focal length is the sum of the two, i.e., f=f₁+f₂. Assuming theglass refractive index dispersion follows the Cauchy equation:

${{n_{g}(\lambda)} = {A_{1} + \frac{B_{1}}{\lambda^{2}} + \frac{C_{1}}{\lambda^{4}}}},$

where A₁, B₁ and C₁ are constants, the dispersion of the focal length ofthe diffractive lens behaves approximately as follows:

${{f(\lambda)} \approx {\frac{\lambda_{0}}{2d}\frac{R^{2}}{\left( {{\lambda A_{2}} + \frac{B_{2}}{\lambda} + \frac{C_{2}}{\lambda^{3}}} \right)}}},$

where A₂, B₂ and C₂ are the Cauchy coefficients for the dispersionfunction of the refractive index of the Fresnel lens material.

Thus, the imaging system of the present embodiment is based on combiningthe refractive lens with the diffractive lens, so that the total focallength f_(tot)=f₁(λ)+f₂(λ) is wavelength-independent simply becausef₁(λ) and f₂(λ) have opposite trends with the wavelength. The refractiveand diffractive lenses in their achromatic combination should be of thesame power to minimise the requirement on the refractive indexdispersion relation. It is also possible to choose them both to be madeof the liquid crystal composite of the present invention and tune themso that at each external field the focal length obtained iswavelength-independent in a wide spectral range.

The diffractive element zones maybe optimised both in their width and intheir optical path difference so that each zone is capable of minimisingthe chromatic aberration. Assuming the zones corresponding to onedesigned wavelength are r_(j2)≈√{square root over (jλ₁f₂)}, then it ispossible to introduce other zones between them that would correspond toa different design wavelength r_(j2)≈√{square root over (jλ₂f₂)}. As aresult, the obtained generalised Fresnel lens is suitable for wide rangeof wavelengths. In a particular embodiment of a manufacturing methodshown in FIG. 31 and described below, these zones are created as annuluson the transparent electrodes having widths much less than the designedwidth of the Fresnel zone defined by r_(j+1)−r_(j). Such Fresnel zonestructure provides a further degree of freedom to adjust the phaseprofile, so as to minimise chromatic aberrations, extend the depth offield and avoid the polarisation dependence. Another advantage of havingthe fine structure of each Fresnel zone to be created of many annuli isa possibility to create different phase profiles interlaced with eachother. Each profile can be designed to be appropriate for a specificwavelength region or specific focus region, so that the extended depthof field and achromatic operations is obtained.

The optimum phase profiles can be found using large variety ofoptimisation algorithms known in the art such as neural nets, simulatedannealing, and machine learning. In yet further embodiment, part of thesub-Fresnel zones is nano-patterned with lines in one direction, butother sub-Fresnel zones are patterned with lines in the perpendiculardirection. This is important because the LC molecules orientationfollows the nano-grid pattern direction, and therefore, some regions ofthe Fresnel lens provide focusing for one polarisation (for instance,transverse-electric), and other regions with the perpendicularorientation provide tuneable focusing of the orthogonal polarisation(such as transverse-magnetic). The nano-grid pattern can be created withvariety of lithographic techniques, nanoimprinting, or irradiation withfemto-second- or ultrashort-pulsed lasers. The nano-grid pattern createsanisotropic surface tension which causes the LC molecules to be alignedin one direction. There are other techniques for generating the surfacetension anisotropy, for example using mechanical rubbing in onedirection, depositing material at oblique incidence, coating with blackphosphorous layer or transition metal sulphides, coating the surfacewith photosensitive polymer layer or polyimide, or chalcogenide glass,and then irradiating it with polarised UV or blue light.

Reference is now made to FIG. 31 describing the method for manufacturingthe liquid crystal composite tuneable device of the present invention.In order to make electrical connections to the different electrodes in away that they do not disturb the transmitted light intensity orwave-front, the manufacturing method comprises the following steps:

-   -   (1) Deposition of a wiring pattern made from indium tin oxide or        other TCE material on each of the transparent panels. In this        step, the connecting wires are created on the glass transparent        panel;    -   (2) Deposition of an insulation layer on top of the connecting        wires. In this step, an insulating transparent layer, such as        silicon oxide, is deposited on top of the wires, leaving only        the external edges of the wires exposed for connections;    -   (3) Creation of through holes or blind vias in the insulation        layer near the edges of the connecting wires;    -   (4) Deposition of the transparent conductive electrode layer        made for example from titanium oxide on top of the insulation        layer;    -   (5) Formation of the rings annulus pattern on top of the        transparent conductive electrode layer, so that each ring is        connected to one connecting wire; and    -   (6) Assembly of the LCD of the present invention with the liquid        crystal layer introduced between the two transparent panels.

In yet another embodiment, the annular regions and sub-regions arereplaced with rectangular lines or sub-lines so that the tuneableachromatic lens becomes a tuneable achromatic cylindrical lens. Once canalso simplify the driving by combining two such achromatic cylindricallenses oriented at 90 degrees with respect to each other. Anotheradvantage of using the rectangular regions instead of annular ones isthe ease of manufacturing and electrical connections.

While certain features of the present application have been illustratedand described herein, many modifications, substitutions, changes, andequivalents will be apparent to those of ordinary skill in the art. Itis, therefore, to be understood that the appended claims are intended tocover all such modifications and changes as fall within the true spiritof the present application.

1. A fast wide dynamic range, controlled-bandwidth tuneable filter andmodulator of an incident light beam comprising a liquid crystalcomposite and a reflective planar multi-layer resonant structure, saidtuneable filter and modulator further comprising: (a) a coupling medium;(b) multilayer structure coated either directly on the coupling medium,or on a glass substrate attached to the coupling medium with an indexmatching fluid, said multilayer structure comprising: an ultrathinabsorbing layer acting as a medium to excite a broadband lossy wave; afirst transparent clad layer of low refractive index; a firsttransparent alignment layer; a transparent waveguide layer made oftuneable optical path material either refractive index or thicknesstuneable; a second alignment layer; a second transparent clad layer oflow refractive index; a transparent electrode layer as a secondelectrode layer; and a top transparent semi-infinite substrate of lowrefractive index; characterised in that: (i) said ultrathin absorbinglayer is made of highly absorbing metal, semiconductor, or semimetal soit can act both as a first electrode in addition to being a medium forexciting the broadband lossy wave. (ii) said first transparent cladlayer of low refractive index is configured to control the bandwidth ofthe filter by varying its thickness; (iii) said tuneable refractiveindex waveguide layer is a liquid crystal composite with the porousmicroparticles of less than 0.1% concentration to prevent significantscattering and provide fast refractive index modulation; (iv) saidrefractive index waveguide layer is made of a thermooptic, electrooptic,photosensitive or magnetooptic material; (v) said electrode layers arepatterned into pixels, and an external field is applied to each pixelseparately or to a group of pixels, thereby resulting in said devicebeing pixelated, and said each pixel is adjusted to modify wavelength,intensity, or phase of an optical beam, thereby turning said liquidcrystal composite tuneable device into a spatial light modulator.
 2. Thetuneable filter and modulator of claim 1, wherein said coupling mediumis a high refractive index prism, inline prism, grating or opticalfibre.
 3. The tuneable filter and modulator of claim 1, wherein theliquid crystal is in any of the transparent non scattering modes suchas: nematic, twisted, vertically aligned, planarly aligned, in-planeswitching mode, hybrid, smectic, chiral smectic, cholesteric, bluephase, nanoPDLC, dual frequency, ferroelectric, anti-ferroelectric,flexoelectric, electroclinic, conical, polymeric or ferronematic LCs. 4.The tuneable filter and modulator of claim 1, wherein the tuneablerefractive index waveguide material is made of solid thin filmelectrooptic crystal, electrooptic polymer, piezoelectric ormagnetooptic film, simplified by removing the top low index substrate.5. The tuneable filter and modulator of claim 1 is tuned by angularscanning, wherein the structure is simplified by having the waveguidelayer not limited to tuneable thermooptic, electrooptic, photosnesitiveor magnetooptic and no need for the second cladding, the secondelectrode and the top refractive low index substrate.
 6. The tuneablefilter and modulator of claim 1, wherein the incident light beam isdiverging in the incidence plane so that a rainbow of different coloursor wavelengths is obtained at the output, each wavelength can then bedetected in parallel using an array of detectors or using singledetector through slit and mechanical scanning.
 7. The tuneable filter ormodulator of claim 1 further comprising a polarisation independentconfiguration to obtain a polarisation-independent modulator andtuneable filter.
 8. The tuneable filter or modulator of claim 1, whereinfirst clad layer is absent resulting in the tuneable filter or modulatorwithout control of the bandwidth.
 9. The tuneable filter and modulatorof claim 1, wherein the difference between the refractive indices of thewaveguide layer and the cladding layer is small enough, and thewaveguide layer thickness is large enough (above few wavelengths) togenerate Rabi splitting for higher sensitivity and blue shift of theresonance, as the waveguide optical path increases thus allowing the useof fast materials, fast thermooptic, fast electrooptic, fastmagnetooptic, fast photosensitive or fast piezoelectric effects thatprovide small refractive index or thickness modulation to generate widetuning range.
 10. The tuneable filter and modulator of claim 9, whereinthe difference between the refractive indices of the waveguide layer andthe cladding layer is less than 0.1, and the waveguide layer thicknessis above few wavelengths.
 11. The tuneable filter and modulator of claim1 having large enough waveguide layer thickness to generate multiplereflection peaks within the spectral range of interest so to act asspectral modulator for hyperspectral or multispectral informationextraction when combined with computational spectroscopy techniques. 12.The tuneable filter and modulator of claim 11, wherein the waveguidelayer thickness is more than few wavelengths to generate multiplereflection peaks within the spectral range of interest so to act asspectral modulator for hyperspectral or multispectral informationextraction when combined with computational spectroscopy techniques. 13.The tuneable filter and modulator of claim 1 acting as opticallyaddressed spatial light modulator with the waveguide layer being made ofphotosensitive material to one wavelength range not experiencing theresonant reflection to modulate the resonantly reflected waves ofdifferent spectral range in where the photosensitive layer istransparent.
 14. The tuneable filter and modulator of claim 1 acting asoptically addressed spatial light modulator with the waveguide layerbeing adjacent to a layer photosensitive to one wavelength range notexperiencing the resonant reflection to modulate the resonantlyreflected waves of different spectral range in where the photosensitivelayer is transparent.